alpha particle of experiment

What is the 'Gold Foil Experiment'? The Geiger-Marsden experiments explained

Physicists got their first look at the structure of the atomic nucleus.

The gold foil experiments gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world.

J.J. Thomson model of the atom

Gold foil experiments, rutherford model of the atom.

The real atomic model

Additional Resources

Bibliography.

The Geiger-Marsden experiment, also called the gold foil experiment or the α-particle scattering experiments, refers to a series of early-20th-century experiments that gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world. It was first proposed by Nobel Prize -winning physicist Ernest Rutherford.

As familiar as terms like electron, proton and neutron are to us now, in the early 1900s, scientists had very little concept of the fundamental particles that made up atoms .

In fact, until 1897, scientists believed that atoms had no internal structure and believed that they were an indivisible unit of matter. Even the label "atom" gives this impression, given that it's derived from the Greek word "atomos," meaning "indivisible."

But that year, University of Cambridge physicist Joseph John Thomson discovered the electron and disproved the concept of the atom being unsplittable, according to Britannica . Thomson found that metals emitted negatively charged particles when illuminated with high-frequency light.

His discovery of electrons also suggested that there were more elements to atomic structure. That's because matter is usually electrically neutral; so if atoms contain negatively charged particles, they must also contain a source of equivalent positive charge to balance out the negative charge.

By 1904, Thomson had suggested a "plum pudding model" of the atom in which an atom comprises a number of negatively charged electrons in a sphere of uniform positive charge, distributed like blueberries in a muffin.

The model had serious shortcomings, however — primarily the mysterious nature of this positively charged sphere. One scientist who was skeptical of this model of atoms was Rutherford, who won the Nobel Prize in chemistry for his 1899 discovery of a form of radioactive decay via α-particles — two protons and two neutrons bound together and identical to a helium -4 nucleus, even if the researchers of the time didn't know this.

Rutherford's Nobel-winning discovery of α particles formed the basis of the gold foil experiment, which cast doubt on the plum pudding model. His experiment would probe atomic structure with high-velocity α-particles emitted by a radioactive source. He initially handed off his investigation to two of his protégés, Ernest Marsden and Hans Geiger, according to Britannica .

Rutherford reasoned that if Thomson's plum pudding model was correct, then when an α-particle hit a thin foil of gold, the particle should pass through with only the tiniest of deflections. This is because α-particles are 7,000 times more massive than the electrons that presumably made up the interior of the atom.

Here, an illustration of Rutherford's particle scattering device used in his gold foil experiment.

Marsden and Geiger conducted the experiments primarily at the Physical Laboratories of the University of Manchester in the U.K. between 1908 and 1913.

The duo used a radioactive source of α-particles facing a thin sheet of gold or platinum surrounded by fluorescent screens that glowed when struck by the deflected particles, thus allowing the scientists to measure the angle of deflection.

The research team calculated that if Thomson's model was correct, the maximum deflection should occur when the α-particle grazed an atom it encountered and thus experienced the maximum transverse electrostatic force. Even in this case, the plum pudding model predicted a maximum deflection angle of just 0.06 degrees.

Of course, an α-particle passing through an extremely thin gold foil would still encounter about 1,000 atoms, and thus its deflections would be essentially random. Even with this random scattering, the maximum angle of refraction if Thomson's model was correct would be just over half a degree. The chance of an α-particle being reflected back was just 1 in 10^1,000 (1 followed by a thousand zeroes).

Yet, when Geiger and Marsden conducted their eponymous experiment, they found that in about 2% of cases, the α-particle underwent large deflections. Even more shocking, around 1 in 10,000 α-particles were reflected directly back from the gold foil.

Rutherford explained just how extraordinary this result was, likening it to firing a 15-inch (38 centimeters) shell (projectile) at a sheet of tissue paper and having it bounce back at you, according to Britannica

Extraordinary though they were, the results of the Geiger-Marsden experiments did not immediately cause a sensation in the physics community. Initially, the data were unnoticed or even ignored, according to the book "Quantum Physics: An Introduction" by J. Manners.

The results did have a profound effect on Rutherford, however, who in 1910 set about determining a model of atomic structure that would supersede Thomson's plum pudding model, Manners wrote in his book.

The Rutherford model of the atom, put forward in 1911, proposed a nucleus, where the majority of the particle's mass was concentrated, according to Britannica . Surrounding this tiny central core were electrons, and the distance at which they orbited determined the size of the atom. The model suggested that most of the atom was empty space.

When the α-particle approaches within 10^-13 meters of the compact nucleus of Rutherford's atomic model, it experiences a repulsive force around a million times more powerful than it would experience in the plum pudding model. This explains the large-angle scatterings seen in the Geiger-Marsden experiments.

Later Geiger-Marsden experiments were also instrumental; the 1913 tests helped determine the upper limits of the size of an atomic nucleus. These experiments revealed that the angle of scattering of the α-particle was proportional to the square of the charge of the atomic nucleus, or Z, according to the book "Quantum Physics of Matter," published in 2000 and edited by Alan Durrant.

In 1920, James Chadwick used a similar experimental setup to determine the Z value for a number of metals. The British physicist went on to discover the neutron in 1932, delineating it as a separate particle from the proton, the American Physical Society said .

What did the Rutherford model get right and wrong?

Yet the Rutherford model shared a critical problem with the earlier plum pudding model of the atom: The orbiting electrons in both models should be continuously emitting electromagnetic energy, which would cause them to lose energy and eventually spiral into the nucleus. In fact, the electrons in Rutherford's model should have lasted less than 10^-5 seconds.

Another problem presented by Rutherford's model is that it doesn't account for the sizes of atoms.

Despite these failings, the Rutherford model derived from the Geiger-Marsden experiments would become the inspiration for Niels Bohr 's atomic model of hydrogen , for which he won a Nobel Prize in Physics .

Bohr united Rutherford's atomic model with the quantum theories of Max Planck to determine that electrons in an atom can only take discrete energy values, thereby explaining why they remain stable around a nucleus unless emitting or absorbing a photon, or light particle.

Thus, the work of Rutherford, Geiger (who later became famous for his invention of a radiation detector) and Marsden helped to form the foundations of both quantum mechanics and particle physics.

Rutherford's idea of firing a beam at a target was adapted to particle accelerators during the 20th century. Perhaps the ultimate example of this type of experiment is the Large Hadron Collider near Geneva, which accelerates beams of particles to near light speed and slams them together.

See a modern reconstruction of the Geiger-Marsden gold foil experiment conducted by BackstageScience and explained by particle physicist Bruce Kennedy .
Find out more about the Bohr model of the atom which would eventually replace the Rutherford atomic model.
Rutherford's protege Hans Gieger would eventually become famous for the invention of a radioactive detector, the Gieger counter. SciShow explains how they work .

Thomson's Atomic Model , Lumens Chemistry for Non-Majors,.

Rutherford Model, Britannica, https://www.britannica.com/science/Rutherford-model

Alpha particle, U.S NRC, https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html

Manners. J., et al, 'Quantum Physics: An Introduction,' Open University, 2008.

Durrant, A., et al, 'Quantum Physics of Matter,' Open University, 2008

Ernest Rutherford, Britannica , https://www.britannica.com/biography/Ernest-Rutherford

Niels Bohr, The Nobel Prize, https://www.nobelprize.org/prizes/physics/1922/bohr/facts/

House. J. E., 'Origins of Quantum Theory,' Fundamentals of Quantum Mechanics (Third Edition) , 2018

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Robert Lea is a science journalist in the U.K. who specializes in science, space, physics, astronomy, astrophysics, cosmology, quantum mechanics and technology. Rob's articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University

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The rutherford-geiger-marsden experiment.

April 11, 2017 Alpha Spectroscopy , English Posts 86,568 Views

What made by Rutherford and his assistants Geiger and Marsden is perhaps one of the most important experiments of nuclear physics.

The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms).

Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as pudding model ) or if there was something different.

In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with large angle of deviation . Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a small and compact nucleus , but with high charge density, surrounded by an electron cloud. In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil; one can see how the majority of the particles passes undisturbed, or with small angles of deflection, through the “empty” atom, some particles, however, passing close to the nucleus are diverted with a high angle or even bounced backwards.

The interaction between an alpha particle and the nucleus (elastic collision) is also known as Coulomb scattering , because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.

Experimental Setup

In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in alpha spectroscopy we built a setup based on an alpha solid-state detector , a 0.9 μCi Am 241 source and a gold foil as a scatterer. In these post we describe the equipment used : Alpha Spectrometer , Gold Leaf Thickness . The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection. The images below show the experimental setup:

The alpha source is actually 0.9 μCi of Am 241 (from smoke detector) which emits alpha particles with energy of 5.4 MeV. The alpha particle beam is collimated by a simple hole in a wooden screen. Source and collimator are fixed on a arm free to rotate around a pivot, which hosts the gold foil that acts as a scatterer. The whole is placed inside a sealed box that acts as a vacuum chamber with the help of an ordinary oil rotary vacuum pump. The images below show the “vacuum chamber” and the electronic part for amplification and acquisition connected to the PC for counting events.

Linear Scale :

Semilog Scale

The results obtained in our experiment approach, albeit with obvious limitations, to the expected theoretical results, represented in the following graph:

For completeness, we report also at the side the formula that describes the distribution of the number of the counted particles in function of the scattering angle. Interestingly, this depends on the power of two the atomic number of the target and is inversely proportional to the fourth power of the sin (θ/2).

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Tags Alpha spectrometer Rutherford

Gamma Spectroscopy with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In the previous post, we described the apparatus in general terms. Now we mainly focus on the gamma spectrometer functionality.

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Rutherford Scattering

Michael Fowler, University of Virginia

Rutherford as Alpha-Male

[Rutherford was] a "tribal chief", as a student said.

(Richard Rhodes, The Making of the Atomic Bomb, page 46)

In 1908 Rutherford was awarded the Nobel Prize—for chemistry! The award citation read: "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances." While at McGill University, he had discovered that the radioactive element thorium emitted a gas which was itself radioactive, but if the gas radioactivity was monitored separately from the thorium's, he found it decreased geometrically, losing approximately half its current strength for each minute that passed. The gas he had found was a short-lived isotope of radon, and this was the first determination of a "half-life" for a radioactive material. (Pais, Inward Bound , page 120).

The chemists were of course impressed that Rutherford was fulfilling their ancient alchemical dream of transmuting elements, or at least demonstrating that it happened. Rutherford himself remarked at the ceremony that he "had dealt with many different transformations with various time-periods, but the quickest he had met was his own transformation from a physicist to a chemist". Still, Nobel prizes of any kind are nice to get, so he played along, titling his official Nobel lecture: "The chemical nature of the alpha-particle from radioactive substances". (He established that his favorite particle was an ionized helium atom by collecting alphas in an evacuated container, where they picked up electrons. After compressing this very rarefied gas, he passed an electric discharge through it and observed the characteristic helium spectrum in the light emitted.)

Rutherford was the world leader in alpha-particle physics. In 1906, at McGill University, Montreal, he had been the first to detect slight deflections of alphas on passage through matter. In 1907, he became a professor at the University of Manchester, where he worked with Hans Geiger . This was just a year after Rutherford's old boss, J. J. Thomson , had written a paper on his plum pudding atomic model suggesting that the number of electrons in an atom was about the same as the atomic number. (Not long before, people had speculated that atoms might contain thousands of electrons. They were assuming that the electrons contributed a good fraction of the atom's mass.) The actual distribution of the electrons in the atom, though, was as mysterious as ever. Mayer's floating magnets (see previous lecture) were fascinating, but had not led to any quantitative conclusions on electronic distributions in atoms.

Rutherford's 1906 discovery that his pet particles were slightly deflected on passing through atoms came about when he was finding their charge to mass ratio, by measuring the deflection in a magnetic field. He detected the alphas by letting them impact photographic film. When he had them pass through a thin sheet of mica before hitting the film (so the film didn't have to be in the vacuum?) he found the image was blurred at the edges, evidently the mica was deflecting the alphas through a degree or two. He also knew that the alphas wouldn't be deflected a detectable amount by the electrons in the atom, since the alphas weighed 8,000 times as much as the electrons, atoms contained only a few dozen electrons, and the alphas were very fast. The mass of the atom must be tied up somehow with the positive charge . Therefore, he reasoned, analyzing these small deflections might give some clue as to the distribution of positive charge and mass in the atom, and therefore give some insight into his old boss J. J.'s plum pudding. The electric fields necessary in the atom for the observed scattering already seemed surprisingly high to Rutherford (Pais, page 189).

Scattering Alphas

Rutherford's alpha scattering experiments were the first experiments in which individual particles were systematically scattered and detected. This is now the standard operating procedure of particle physics. To minimize alpha loss by scattering from air molecules, the experiment was carried out in a fairly good vacuum, the metal box being evacuated through a tube T (see below). The alphas came from a few milligrams of radium (to be precise, its decay product radon 222) at R in the figure below, from the original paper, which goes on:

" By means of a diaphragm placed at D, a pencil of alpha particles was directed normally on to the scattering foil F. By rotating the microscope [M] the alpha particles scattered in different directions could be observed on the screen S."

Actually, this was more difficult than it sounds. A single alpha caused a slight fluorescence on the zinc sulphide screen S at the end of the microscope. This could only be reliably seen by dark-adapted eyes (after half an hour in complete darkness) and one person could only count the flashes accurately for one minute before needing a break, and counts above 90 per minute were too fast for reliability. The experiment accumulated data from hundreds of thousands of flashes.

Rutherford's partner in the initial phase of this work was Hans Geiger, who later developed the Geiger counter to detect and count fast particles. Many hours of staring at the tiny zinc sulphide screen in the dark must have focused his mind on finding a better way!

In 1909, an undergraduate, Ernest Marsden, was being trained by Geiger. To quote Rutherford (a lecture he gave much later):

"I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree.

"One day Geiger came to me and said, "Don't you think that young Marsden , whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?"

"I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Disproof of the Pudding

The back scattered alpha-particles proved fatal to the plum pudding model. A central assumption of that model was that both the positive charge and the mass of the atom were more or less uniformly distributed over its size, approximately 10 -10 meters across or a little more. It is not difficult to calculate the magnitude of electric field from this charge distribution. (Recall that this is the field that must scatter the alphas, the electrons are so light they will jump out of the way with negligible impact on an alpha.)

To be specific, let us consider the gold atom, since the foil used by Rutherford was of gold, beaten into leaf about 400 atoms thick. The gold atom has a positive charge of 79 e (balanced of course by that of the 79 electrons in its normal state). Neglect the electrons—they'll be scattered away with negligible impact on the heavy alpha.

See the animation here !

The maximum electric force the alpha will encounter is that at the surface of the sphere of positive charge,

E ⋅ 2 e = 1 4 π ε 0 ⋅ 79 e ⋅ 2 e r 0 2 = 9 ⋅ 10 9 158 ⋅ ( 1.6 ⋅ 10 − 19 ) 10 − 20 = 3.64 ⋅ 10 − 6 Newtons .

(In this model, once inside the sphere the electric force goes down, just as gravity goes down on going deep into the earth, to zero at the center. But the sideways component stays approximately constant if the path is nearly a straight line.)

If the alpha particle initially has momentum p , for small deflections the angle of deflection (in radians) is given by Δ p / p , where Δ p is the sideways momentum resulting from the electrically repulsive force of the positive sphere of charge.

A good estimate of the sideways deflection is given by taking the alpha to experience the surface force given above for a time interval equal to the time it takes the alpha to cross the atom—say, a distance 2 r 0 . (The force felt when outside the ball of charge is much smaller: it drops away as the inverse square, but at an angle that makes it effectively inverse cube. It can be shown to make only a small contribution.)

Note that since the alpha particle has mass 6.7x10 -27 kg, from F = m a , the electric force at the atomic surface above will give it a sideways acceleration of 5.4x10 20 meters per sec per sec (compare g = 10 !). But the force doesn't have long to act—the alpha is moving at 1.6x10 7 meters per second. So the time available for the force to act is the time interval a particle needs to cross an atom if the particle gets from New York to Australia in one second.

So the transit time for the alpha across the plum pudding atom is:

t 0 = 2 r 0 / v = 2 × 10 10 / 1.6 × 10 7 = 1.25 × 10 − 17 seconds .

Now, the magnitude of the total sideways velocity picked up on crossing the atom is the sideways acceleration multiplied by the time,

1.25 × 10 − 17 × 5.4 × 10 20 = 6750 m /sec .

This is a few ten-thousandths of the alpha's forward speed , so there is only a very tiny deflection . Even if the alpha hit 400 atoms in succession and they all deflected it the same way, an astronomically improbable event, the deflection would only be of order a degree. Therefore, the observed deflection through ninety degrees and more was completely inexplicable using Thomson's pudding model!

Emergence of the Nucleus

Rutherford pondered the problem for some months. He had been a believer in his former boss's pudding model, but he eventually decided there was simply no way it could generate the strength of electric field necessary to deflect the fast moving alphas. Yet it was difficult to credit there was much more positive charge around than that necessary to compensate for the electrons, and it was pretty well established that there were not more than a hundred or so electrons (we used 79, the correct value—that was not known exactly until a little later). The electric field from a sphere of charge reaches its maximum on the surface, as discussed above. Therefore, for a given charge, assumed spherically distributed, the only way to get a stronger field is to compress it into a smaller sphere . Rutherford concluded that he could only explain the large alpha deflections if the positive charge, and most of the mass of the atom, was in a sphere much smaller than the atom itself .

It is not difficult to estimate from the above discussion how small such a nucleus would have to be to give a substantial deflection. We found a sphere of radius 10 -10 meters gave a deflection of about 4x10 -4 radians. We need to increase this deflection by a factor of a few thousand. On decreasing the radius of the sphere of positive charge, the force at the surface increases as the inverse radius squared . On the other hand, the time over which the alpha experiences the sideways force decreases as the radius.

The total deflection , then, proportional to the product of force and time, increases as the inverse of the radius . This forces the conclusion that the positive charge is in a sphere of radius certainly less than 10 -13 meters, provided all the observed scattering is caused by one encounter with a nucleus.

Animation of scattering from a nuclear atom here !

Rutherford decided that the observed scattering was in fact from a single nucleus. He argued as follows: since the foil is only 400 atoms thick, it is difficult to see how ninety degree scatterings could arise unless the scattering by a single nucleus was at least one degree, say 100 times that predicted by the Thomson model. This would imply that the nucleus had a radius at most one-hundredth that of the atom, and therefore presented a target area for one-degree scattering (or more) to the incoming alphas only one ten-thousandth that of the atom. (In particle physics jargon, this target area is called the scattering cross section .) If an alpha goes through 400 layers of atoms, and in each layer it has a chance of one in ten thousand of getting close enough to the nucleus for a one-degree scatter, this is unlikely to happen twice. It follows that almost certainly only one scattering takes place. It then follows that all ninety or more degrees of scattering must be a single event, so the nucleus must be even smaller than one hundredth the radius of the atom -- it must be less than 10 -13 meters, as stated above.

Seeing the Nucleus

Having decided that the observed scattering of the alphas came from single encounters with nuclei, and assuming that the scattering force was just the electrostatic repulsion, Rutherford realized maybe just scaling down the radius in the plum pudding analysis given above wasn't quite right. Maybe the nucleus was so small that the alpha particle didn't even touch it. If that were the case, the alpha particle's entire trajectory was determined by a force law of inverse square repulsion, and could be analyzed precisely mathematically by the techniques already well-known to astronomers for finding paths of planets under inverse square attraction.

It turns out that the alpha will follow a hyperbolic path (see the animation). Imagine an alpha coming in along an almost straight line path, the perpendicular distance of the nucleus from this line is called the impact parameter (how close to the center the alpha particle would pass if the repulsion were switched off). The standard planetary math is enough to find the angle at which the alpha comes out (the scattering angle), given the impact parameter and speed. Although not exactly a hot shot theorist, Rutherford managed to figure this out after a few weeks.

The incoming stream of alphas all have the same velocity (including direction) , but random impact parameters: we assume the beam intensity doesn't vary much in the perpendicular direction, certainly on an atomic scale, so we average over impact parameters (with a factor 2 π p d p for the annular region p , p + d p ).

The bottom line is that for a nucleus of charge Z , and incident alpha particles of mass m and speed v , the rate of scattering to a point on the screen corresponding to a scattering angle of θ (angle between incident velocity and final velocity of alpha) is proportional to:

scattering into small area at θ ∝ ( 1 4 π ε 0 ⋅ Z e 2 m v 2 ) 2 ⋅ 1 sin 4 ( θ / 2 ) .

Analysis of the hundred thousand or more scattering events recorded for the alphas on gold fully confirmed the angular dependence predicted by the above analysis.

Modeling the Scattering

To visualize the path of the alpha in such a scattering, Rutherford "had a model made, a heavy electromagnet suspended as a pendulum on thirty feet of wire that grazed the face of another electromagnet set on a table. With the two grazing faces matched in polarity and therefore repelling each other, the pendulum was deflected" into a hyperbolic path.(Rhodes, page 50)

But it didn't work for Aluminum...

On replacing the gold foil by aluminum foil (some years later), it turned out that small angle scattering obeyed the above law, but large angle scattering didn't. Rutherford correctly deduced that in the large angle scattering, which corresponded to closer approach to the nucleus, the alpha was actually hitting the nucleus. This meant that the size of the nucleus could be worked out by finding the maximum angle for which the inverse square scattering formula worked, and finding how close to the center of the nucleus such an alpha came. Rutherford estimated the radius of the aluminum nucleus to be about 10 -14 meters.

The Beginnings of Nuclear Physics

The First World War lasted from 1914 to 1918. Geiger and Marsden were both at the Western front, on opposite sides. Rutherford had a large water tank installed on the ground floor of the building in Manchester, to carry out research on defense against submarine attack. Nevertheless, occasional research on alpha scattering continued. Scattering from heavy nuclei was fully accounted for by the electrostatic repulsion, so Rutherford concentrated on light nuclei, including hydrogen and nitrogen. In 1919, Rutherford established that an alpha impinging on a nitrogen nucleus can cause a hydrogen atom to appear! Newspaper headlines blared that Rutherford had "split the atom". (Rhodes, page 137)

Shortly after that experiment, Rutherford moved back to Cambridge to succeed J. J. Thomson as head of the Cavendish laboratory, working with one of his former students, James Chadwick , who had spent the war years interned in Germany. They discovered many unusual effects with alpha scattering from light nuclei. In 1921, Chadwick and co-author Bieler wrote: "The present experiments do not seem to throw any light on the nature of the law of variation of the forces at the seat of an electric charge, but merely show that the forces are of great intensity … It is our task to find some field of force which will reproduce these effects." I took this quote from Pais, page 240, who goes on to say that he considers this 1921 statement as marking the birth of the strong interactions.

In fact, Rutherford was beginning to focus his attention on the actual construction of the nucleus and the alpha particle. He coined the word "proton" to describe the hydrogen nucleus, it first appeared in print in 1920 (Pais). At first, he thought the alpha must be made up of four of these protons somehow bound together by having two electrons in the middle—this would get the mass and charge right, but of course nobody could construct a plausible electrostatic configuration. Then he had the idea that maybe there was a special very tightly bound state of a proton and an electron, much smaller than an atom. By 1924, he and Chadwick were discussing how to detect this neutron. It wasn't going to be easy—it probably wouldn't leave much of a track in a cloud chamber. In fact, Chadwick did discover the neutron, but not until 1932, and it wasn't much like their imagined proton-electron bound state. But it did usher in the modern era in nuclear physics.

Misconceptions

Classroom Physics

Alpha particle scattering

Demonstration

Rutherford’s scattering experiment was an ingenious piece of design and interpretation. Whilst it is not possible to reproduce the experiment in a school laboratory, it is well worth demonstrating how it was carried out using photographs, pictures and analogies.

Apparatus and Materials

Pictures of Rutherford scattering (for example the two further down the page)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Originally the scintillations were counted by eye: trained observers counted for a short time in a darkened room. Rutherford is reputed to have sung Onward Christian Soldiers as he waited for the next flash of light! Today, modern detectors, such as photomuliplier tubes connected to a data logger, would be used.

Much later, a similar method (deep inelastic scattering of electrons) was used to probe inside neutrons and protons and determine their structure: they are made of three smaller particles called quarks.

It is helpful to show students a video of the alpha scattering experiment and to emphasize that this is not just one more measurement in atomic physics but rather one of the great turning points in physics. It changed scientists' picture of atoms permanently. (You may be able to get hold of a second-hand copy of the Nuffield A-Level version.)
Show students a picture of the layout of a scattering experiment showing alpha particles being fired at a ‘solid’ gold foil.

Developing a model of the atom: the nuclear atom

Teaching Notes

99.99% of alpha particles are undeflected. This implies that the atom is mainly hollow.
Some alpha particles bounce back. This implies that there is a single structure in the atom which is more massive than the alpha particle and (probably) repels it.
More careful studies carried out by Geiger and Marsden provided results that were consistent with a single nucleus carrying a positive charge +Ze where Z is the atomic number of the scattering atom.

Students need help with these ideas. Analogies such as the three mentioned below will help.
A haystack analogy: Suppose you wished to investigate the shape and size of a concealed object. Pretend you have a large mound or truss of hay and you suspect that there are some small but massive iron cannon balls hidden in it. Suppose you are not allowed to pull the hay aside. You might still investigate its secret store by firing a stream of machine-gun bullets into it. Let us make some predictions.

If the hay is only a thin covering on a solid mound of cannon balls then all the bullets will bounce back or perhaps stop and never reappear.
If there is only hay, and no cannon balls, then the bullets will go completely through the bale of hay, moving almost as fast as they went in.
However if the mound was mostly hay with a few dense cannon balls scattered through it, spaced well apart, then many of the bullets will go straight through it and a few will bounce back. The proportion bouncing back indicates the spacing of the 'nuclei' in the hay. The more that bounce back, the closer together are the canon balls. To realistically model the nuclei in gold foil, 10 cm canon balls would need a distance of 10 km between them. This illustrates how tiny a nucleus is and how hollow an atom is.
Marbles analogy: Another model might include marbles rolling down a slightly sloping table with a few spikes sticking out such as in a pin-ball machine. What would happen to the marbles?
Experimental analogies: You can demonstrate magnetic, electrostatic and gravitational models in the experiments listed below. Rutherford himself used the magnetic model and analogy to explain his theory.

Rutherford's alpha scattering experiment

The great scattering experiments

Developing a model of the atom: a nuclear atom

This experiment was safety-checked in December 2006

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Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom

In 1911, Rutherford, along with his assistants, H. Geiger and E. Marsden, performed the Alpha Particle scattering experiment , which led to the birth of the ‘nuclear model of an atom ’ – a major step towards how we see the atom today.

Browse more Topics under Atoms

Atomic Spectra
Bohr Model of the Hydrogen Atom

The Alpha Particle Scattering Experiment

They took a thin gold foil having a thickness of 2.1×10 -7 m and placed it in the centre of a rotatable detector made of zinc sulfide and a microscope. Then, they directed a beam of 5.5MeV alpha particles emitted from a radioactive source at the foil. Lead bricks collimated these alpha particles as they passed through them.

After hitting the foil, the scattering of these alpha particles could be studied by the brief flashes on the screen. Rutherford and his team expected to learn more about the structure of the atom from the results of this experiment.

Source: Wikipedia

Observations

Here is what they found:

Most of the alpha particles passed through the foil without suffering any collisions
Around 0.14% of the incident alpha particles scattered by more than 1 o
Around 1 in 8000 alpha particles deflected by more than 90 o

These observations led to many arguments and conclusions which laid down the structure of the nuclear model on an atom.

Conclusions and arguments

The results of this experiment were not in sync with the plum-pudding model of the atom as suggested by Thomson. Rutherford concluded that since alpha particles are positively charged, for them to be deflected back, they needed a large repelling force. He further argued that for this to happen, the positive charge of the atom needs to be concentrated in the centre, unlike scattered in the earlier accepted model.

Hence, when the incident alpha particle came very close to the positive mass in the centre of the atom, it would repel leading to a deflection. On the other hand, if it passes through at a fair distance from this mass, then there would be no deflection and it would simply pass through.

He then suggested the ‘nuclear model of an atom’ wherein the entire positive charge and most of the mass of the atom is concentrated in the nucleus. Also, the electrons are moving in orbits around the nucleus akin to the planets and the sun. Further, Rutherford also concluded from his experiments that the size of the nucleus is between 10 -15 and 10 -14 m.

According to Kinetic theory, the size of an atom is around 10 -10 m or around 10,000 to 100,000 times the size of the nucleus proposed by Rutherford. Hence, the distance of the electrons from the nucleus should be around 10,000 to 100,000 times the size of the nucleus.

This eventually implies that most of the atom is empty space and explains why most alpha particles went right through the foil. And, these particles are deflected or scattered through a large angle on coming close to the nucleus. Also, the electrons having negligible mass, do not affect the trajectory of these incident alpha particles.

Alpha Particle Trajectory

The trajectory traced by an alpha particle depends on the impact parameter of the collision. The impact parameter is simply the perpendicular distance of each alpha particle from the centre of the nucleus. Since in a beam all alpha particles have the same kinetic energy, the scattering of these particles depends solely on the impact parameter.

Hence, the particles with a small impact parameter or the particles closer to the nucleus, experience large angle of scattering. On the other hand, those with a large impact parameter suffer no deflection or scattering at all. Finally, those particles having ~zero impact parameter or a head-on collision with the nucleus rebound back.

Coming to the experiment, Rutherford and his team observed that a really small fraction of the incident alpha particles was rebounding back. Hence, only a small number of particles were colliding head-on with the nucleus. This, subsequently, led them to believe that the mass of the atom is concentrated in a very small volume.

Electron Orbits

In a nutshell, Rutherford’s nuclear model of the atom describes it as:

A small and positively charged nucleus at the centre
Surrounded by revolving electrons in their dynamically stable orbits

The centripetal force that keeps the electrons in their orbits is an outcome of:

The positively charged nucleus and
The negatively charged revolving electrons.

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Question: Rutherford, Geiger and Marsden, directed a beam of alpha particles on a foil of which metal

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National Center for Biotechnology Information - PubMed Central - Tracking down alpha-particles: the design, characterisation and testing of a shallow-angled alpha-particle irradiator
Nature - Scientific Reports - Development of an ultrahigh resolution real time alpha particle imaging system for observing the trajectories of alpha particles in a scintillator
Space.com - Alpha particles and alpha radiation: Explained

alpha particle , positively charged particle, identical to the nucleus of the helium -4 atom , spontaneously emitted by some radioactive substances, consisting of two protons and two neutrons bound together, thus having a mass of four units and a positive charge of two. Discovered and named (1899) by Ernest Rutherford , alpha particles were used by him and coworkers in experiments to probe the structure of atoms in thin metallic foils. This work resulted in the first concept of the atom as a tiny planetary system with negatively charged particles (electrons) orbiting around a positively charged nucleus (1909–11). Later, Patrick Blackett bombarded nitrogen with alpha particles, changing it to oxygen , in the first artificially produced nuclear transmutation (1925). Today, alpha particles are produced for use as projectiles in nuclear research by ionization—i.e., by stripping both electrons from helium atoms—and then accelerating the now positively charged particle to high energies.

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Rutherford’s Alpha Scattering Experiment

Rutherford’s Alpha Scattering Experiment is the fundamental experiment done by Earnest Rutherford’s Alpha Scattering Experiment that gives the fundamental about the structure of the atom. Rutherford in his experiment directed high-energy streams of α-particles from a radioactive source at a thin sheet (100 nm thickness) of gold. Then the deflection of these alpha particles tells us about the structure of atoms.

In this article, we will study about constituents of atoms, Rutherford’s Alpha Scattering Experiment,

What are Constituents of an Atom?

An atom consists of Electrons, Protons, and Neutrons are the fundamental particles or sub-atomic particles that build the structure of an atom. Let us understand each term.

Electron: In 1897, J. J. Thomson discovered negatively charged particles towards the anode, these rays are emitted by the cathode in a cathode ray experiment. Then these negatively charged particles are proposed as Electrons .
Protons: In 1886, Ernest Goldstein discovered that an anode emitted positively charged particles with a different condition in the same tube, known as Canal rays or as Protons .
Neutrons: A subatomic particle with no charge and a mass equivalent to protons in the nucleus of all atoms was discovered by J. Chadwick. These neutrally charged particles are termed Neutrons .

The image added below shows the structure of an atom.

Learn more about, Atomic Structure

Structure of Atom

Isotopes are the elements that have the same atomic number but different mass. e.g. Isotopes of the Hydrogen atoms are Protium ( 1 H 1 ), Deuterium ( 2 H 1 ) and Tritium( 3 H 1 ). Isotopes of the Carbon atoms are 12 C 6 , 13 C 6 , 14 C 6 .

Isobars are the elements that have different atomic number but have same mass number. e.g. 19 K 40 , 18 Ar 40 , 20 Ca 40 , here all the elements having same mass number hence they are isobars.

He conduct an experiment by bombarding alpha particles into a thin sheet of gold and then notices their interaction with the gold foil and trajectory or path followed by these particles.

In the experiment, Rutherford passes very high streams of alpha-particles from a radioactive source i.e. alpha-particle emitter, at a thin sheet of100 nm thickness of gold. In order to examine the deflection produced by the alpha particles, he placed a screen of fluorescent zinc sulphide around the thin gold foil. Rutherford made certain observations that oppose Thomson’s atomic model.

Observations of Rutherford’s Alpha Scattering Experiment

The observations of Rutherford’s Alpha Scattering Experiment are:

First, he observe that most of the α-particles that are bombarded towards the gold sheet pass away the foil without any deflection, and hence it shows most of the space is empty.
Out of all, some of the α-particles were deflected through the gold sheet by very small angles, and hence it shows the positive charge in an atom is non-uniformly distributed. The positive charge is concentrated in a very small volume in an atom.
Very few of the alpha-particles(1-2%) were deflected back, i.e. only a very less amount of α-particles had nearly 180° angle of deflection. this shows that the volume occupied by the positively charged particles is very small as compared to the total volume of an atom.

Rutherford Atomic Model

Rutherford proposed the atomic structure of elements, on the basis of his experiment. According to Rutherford’s atomic model:

Positively charged particle was concentrated in an extremely small volume and most of the mass of an atom was also in that volume. He called this a nucleus of an atom.
Rutherford proposed that there is negatively charged electrons around the nucleus of an atom. the electron surrounding the nucleus revolves around it in a circular path with very high speed. He named orbits to these circular paths.
Nucleus being a densely concentrated mass of positively charged particles and electrons being negatively charged are held together by a strong force of attraction called electrostatic forces of attraction.

Learn about, Rutherford Atomic Model

Limitations of Rutherford Atomic Model

The Rutherford atomic model is failed to explain certain things.

According to Maxwell, an electron revolving around the nucleus should emit electromagnetic radiation due to accelerated charged particles emit electromagnetic radiation. but Rutherford model says that the electrons revolve around the nucleus in fixed paths called orbits. The radiation would carry energy from the motion which led to the shrinking of orbit. Ultimately electrons would collapse inside the nucleus.
As per the Rutherford model, calculations have shown that an electron would collapse in the nucleus in less than 10 -8 seconds. So Rutherford model has created a high contradiction with Maxwell’s theory and Rutherford later could not explain the stability of an atom.
Rutherford also did not describe the arrangement of electrons in the orbit as one of the other drawbacks of his model.

Regardless of seeing the early atomic models were inaccurate and failed to explain certain experimental results, they were the base for future developments in the world of quantum mechanics.

Sample Questions on Rutherford’s Alpha Scattering Experiment

Some sample questions on Rutherford’s Alpha Scattering Experiment is,

Q1: Represent Element ‘X’ which contains 15 electrons and 16 neutrons.

Atomic number of element = No. of electron = 15 Mass number of element = no. of electrons + no. of neutrons = 15 + 16 = 31 Correct representation of element X is 31 X 15 .

Q2: Name particle and give its location in the atom which has no charge and has a mass nearly equal to that of a proton.

The particle which has no charge and has a mass nearly equal to that of a proton is a neutron and it is present in the nucleus of the atom.

Q3: An atom has both electron attribute negative charge and protons attribute positive charge but why there is no charge?

Positive and negative charges of protons and electrons are equal in magnitude, they cancel the effect of each other. So, the atom as a whole is electrically neutral.

Q4: What is Valency of Sodium Atom (Na)?

The atomic number of sodium = 11. Electronic configuration (2, 8, 1). By losing one electron it gains stability hence its valency is 1.

Q5: Which property do the following pairs show? 209 X 84 and 210 X 84

Atomic number of X is the same hence the pair shows an isotopic property. So, 209 X 84 and 210 X 84 are isotopes.

Dalton’s Atomic Theory Thomson’s Atomic Model Quantum Numbers

Rutherford’s Alpha Scattering Experiment FAQs

What is name of atom which has one electron, one proton and no neutron.

Atom with one electron, one proton and no neutron is Hydrogen, ( 1 H 1 ).

What is Ground State of an Atom?

It is the state of an atom where all the electrons in the atom are in their lowest energy state or levels is called the ground state.

What was Rutherford’s Alpha Particle Scattering Experiment?

Rutherford’s Alpha Particle Scattering Experiment is the fundamental experiment that gives the basic structure of an atom.

What was Conclusion of Rutherford’s Alpha Scattering Experiment?

Conclusion of Rutherford’s Alpha Scattering Experiment is, Atom is largely empty and has a heavy positive-charged body at the center called the nucleus. The central nucleus is positively charged and the negatively-charged electrons revolve around the nucleus.

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EPISODE 289

Cari Cesarotti on the Next Generation of Particle Experiments

As an experimental facility, the Large Hadron Collider at CERN in Geneva has been extraordinarily successful, discovering the Higgs boson and measuring multiple features of particle-physics interactions at unprecedented energies. But to theorists, the results have been somewhat frustrating, as we were hoping to find brand-new phenomena beyond the Standard Model. There is nothing to do but to keep looking, recognizing that we have to choose our methods judiciously. I talk with theoretical physicist Cari Cesarotti about what experimental results the modern particle physicist most looks forward to, and how we might eventually get there, especially through the prospect of a muon collider.

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Blog post with transcript: https://www.preposterousuniverse.com/podcast/2024/09/16/289-cari-cesarotti-on-the-next-generation-of-particle-experiments/

Cari Cesarotti received her Ph.D. in physics from Harvard University. She is currently a postdoctoral fellow at MIT. Her research is on particle phenomenology theory, with an eye toward experimental searches. Among her awards are the Sakurai Dissertation Award in Theoretical Physics from the American Physical Society and the Young Scientist Award at the 14th International Conference on the Identification of Dark Matter.

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Published September 16, 2024 at 12:00 PM UTC
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Episode 289
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New results from the CMS experiment put W boson mass mystery to rest

Physicists on the CMS experiment announce the most elaborate mass measurement of a particle that has captivated the physics community for decades.

Following up on an unexpected measurement by the Collider Detector at Fermilab experiment in 2022, physicists on the Compact Muon Solenoid experiment at the Large Hadron Collider announced today a new mass measurement of the W boson, one of nature’s force-carrying particles.

This new measurement, which is a first for the CMS experiment, uses a new technique that makes it the most elaborate investigation of the W boson’s mass to date. Drawing on nearly a decade of analysis, CMS has found that the W boson’s mass is consistent with predictions, finally putting a multi-year-long mystery to rest.

The final analysis used 300 million events collected from the 2016 run of the LHC, along with 4 billion simulated events. From this dataset, the team reconstructed and then measured the mass from more than 100 million W bosons. They found that the W boson’s mass is 80 360.2 ± 9.9 megaelectron volts (MeV), which is consistent with the Standard Model’s predictions of 80 357 ± 6 MeV. They also ran a separate analysis that cross-checks the theoretical assumptions.

Since the W boson was discovered in 1983, physicists on 10 different experiments have measured its mass.

“The new CMS result is unique because of its precision and the way we determined the uncertainties,” says Patty McBride, a distinguished scientist at the US Department of Energy’s Fermi National Research Laboratory and the former CMS spokesperson. “We’ve learned a lot from CDF and the other experiments who have worked on the W boson mass question. We are standing on their shoulders, and this is one of the reasons why we are able to take this study a big step forward.”

The W boson is one of the cornerstones of the Standard Model, the theoretical framework that describes nature at its most fundamental level. A precise understanding of the W boson’s mass allows scientists to map the interplay of particles and forces, including the strength of the Higgs field and merger of electromagnetism with the weak force, which is responsible for radioactive decay.

“The entire universe is a delicate balancing act,” says Anadi Canepa, deputy spokesperson of the CMS experiment and a senior scientist at Fermilab. “If the W mass is different from what we expect, there could be new particles or forces at play.”

Diagram: Comparison of W boson mass measurements

Comparison measurements of the W boson’s mass with other experiments and the Standard Model prediction. The dot is the measured value and length of the line corresponds to the precision; the shorter the line, the more precise the measurement. Image based on a figure produced by the CMS collaboration.

The new CMS measurement has a precision of 0.01%. This level of precision corresponds to measuring a 4-inch-long pencil to between 3.9996 and 4.0004 inches. But unlike a pencil, the W boson has no physical volume and a mass lighter than a single atom of silver.

“This measurement is extremely difficult to make,” Canepa says. “We need multiple measurements from multiple experiments to cross-check the value.”

The CMS experiment is distinct from the other experiments that have made this measurement because of its compact design, its specialized sensors for fundamental particles called muons, and its extremely strong solenoid magnet, which bends the trajectories of charged particles as they move through the detector. “CMS’s design makes it particularly well-suited for precision mass measurements,” McBride says. “It’s a next-generation experiment.”

Because most fundamental particles are incredibly short-lived, scientists measure their masses by adding up the masses and momenta of everything they decay into. This method works well for particles like the Z boson, another force-carrying particle similar to the W boson, which decays into two muons. But the W boson poses a big challenge because one of its decay products is a tiny fundamental particle called a neutrino.

“Neutrinos are notoriously difficult to measure,” says Josh Bendavid, a scientist at the Massachusetts Institute of Technology who worked on this analysis. “In collider experiments, the neutrino goes undetected, so we can only work with half the picture.”

Working with just half the picture means that the physicists need to be creative. Before running the analysis on real experimental data, the scientists first simulated billions of LHC collisions. “In some cases, we even had to model small deformations in the detector,” Bendavid says. “The precision is high enough that we care about small twists and bends, even if they’re as small as the width of a human hair.”

Physicists also need numerous theoretical inputs, such as what is happening inside the protons when they collide, how the W boson is produced, and how it moves before it decays. “It’s a real art to figure out the impact of theory inputs,” McBride says.

In the past, physicists used the Z boson as a stand-in for the W boson while calibrating their theoretical models. While this method has many advantages, it also adds a layer of uncertainty into the process. “Z and W bosons are siblings, but not twins,” says Elisabetta Manca, a researcher at the University of California Los Angeles and one of the analyzers. “Physicists need to make a few assumptions when extrapolating from the Z to the W, and these assumptions are still under discussion.”

To reduce this uncertainty, CMS researchers developed a novel analysis technique that uses only real W boson data.

“We were able to do this effectively thanks to a combination of a larger data set, the experience we gained from an earlier W boson study, and the latest theoretical developments,” Bendavid says. “This has allowed us to free ourselves from the Z boson as our reference point.”

As part of this analysis, they also examined 100 million tracks from the decays of well-known particles to recalibrate a massive section of the CMS detector until it was an order of magnitude more precise.

“This new level of precision will allow us to tackle critical measurements, such as those involving the W, Z and Higgs boson, with enhanced accuracy,” Manca says.

The most challenging part of the analysis was its time intensiveness, since it required creating a novel analysis technique and developing an incredibly deep understanding of the CMS detector.

“I started this research as a summer student, and now I’m in my third year as a postdoc,” Manca says. “It’s a marathon, not a sprint.”

Editor's note: A version of this article was originally published as a press release by Fermi National Accelerator Laboratory.

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289 | Cari Cesarotti on the Next Generation of Particle Experiments

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0:00:00.0 Sean Carroll: Hello everyone, welcome to the Mindscape Podcast. I'm your host, Sean Carroll. In science, when things are going well, there is an interplay between theory and experiment. Experimenters notice something about the world, theorists rush to offer an explanation or many explanations for those phenomena, the experimenters go out and test the predictions of the theory, they discover even new things, the theorists are called in again, and it keeps going back and forth. But occasionally, in fact, I would say almost never, but sometimes, one can be the victim of one's own success. And that is the story of modern particle physics. In the 50s, 60s, 70s, we were just splashed with all sorts of experimental results that were very puzzling, very interesting, very intriguing, very hard to explain. We put together a theory, the standard model of particle physics. And then something happened that almost never happens in science, which is that we kept collecting data. The experimenters have not slowed down. They've not gotten worse at their job. They're probing the universe in regimes where we have not yet probed it.

0:01:07.3 SC: But the new data is still in line with the standard model, with the theory that we put together back in the 60s and 70s. The capstone of this, of course, was the discovery in 2012 of the Higgs boson, something that was predicted to exist back in the 1960s, with more or less the same properties that we'd predicted it should have back in the 1960s. Despite that success, there are plenty of reasons to think the standard model is not the final answer. Of course, it doesn't include gravity as a fully quantum mechanical theory, but also what is the dark matter? Why are there so many particles? Why do they have the numbers that they have? Why is there more matter than antimatter? A whole bunch of questions that are looming over the standard model. Or to think of it as a physicist would think about it, there are clues that there must be deeper stuff going on than the standard model of particle physics. And despite this wonderful ability of experimentalists to come up with new experiments, looking beyond where they've looked before, it's hard to keep that project up in fundamental particle physics. It's expensive. The time scales are very, very slow.

0:02:12.5 SC: It's a different kind of problem. Every science has its own idiosyncrasies. But in biology, if you're working with a little C. Elegans roundworm, you can go in there and change its genome and make it do something that has literally never been done before, at least as witnessed by human beings. In particle physics, it is very, very difficult to do an experiment that probes into a regime where human beings have never probed before. You need a lot of money, a lot of engineering know-how, and at this day and age, you need a lot of political will. It's a more-than-one-country kind of international collaboration to get this going. So the question is, The Large Hadron Collider was super successful as a machine. They found the Higgs boson, Nobel Prizes were given out. I would like one more Nobel Prize to be given out to the actual experimenters who were responsible for that, but we'll see whether that happens. The question is where to go next. What are we going to do? We're in this weird position, a theory that fits the data, but we know, or we strongly, strongly think that it's not the final theory.

0:03:16.6 SC: So there's different programs on the board, different proposals for what to do. The folks at CERN, which is the home of Large Hadron Collider, would love to build a larger hadron collider or electron collider or something like that on their site. There are people in China who want to build an ultra high energy machine. We're not sure what we're going to do next. Today's guest, Cari Cesarotti, is a particle theorist. So she works on building models of new kinds of particle physics that could then go out and be tested. But she's especially interested in literally the experiments you can do and figuring out what are the best experiments possibly that you can do. So rather than sitting back in the armchair and thinking about quantum gravity and the emergence of space-time, as some people want to do, she wants to make predictions and then go test them in a very, very detailed way. And the particular way that she is most fond of is colliding muons together. Muons were discovered back in the 1930s by Carl Anderson. There's a brief moment. In the 1930s, when Carl Anderson, a physicist at Caltech, had discovered half of the known particles in existence, because before Anderson there was only electrons, protons, and neutrons, and then he discovered the positron, the antiparticle, the electron, as well as the muon and the antimuon.

0:04:34.3 SC: And the muon was the particle that led II Robbie to say, who ordered that? It was not clear what purpose it served. It was not part of the atom or anything like that. Of course, today there's plenty of things that are not part of our everyday human existence, but they're there, out there as particles. Muons are heavier cousins of the electrons, so they have a lot of benefits that electrons have and protons have. They're kind of a happy medium in between them. Massive like a proton, but easy to control like the electron. So why not build a machine? Well, sadly, the muons decay in about a microsecond. That causes some technological challenges, and it's plausible that we are right now at the verge of being able to address those technological challenges, opening up the possibility of building a muon collider and testing physics beyond the standard model of particle physics in a way we've never been able to do before. Cari's going to give us the sales pitch for doing that. I hope you come away convinced that this would be a wonderful idea. So let's go.

0:05:53.4 SC: Cari Cesarotti, welcome to the Mindscape Podcast.

0:05:55.5 Cari Cesarotti: Hi Sean, thanks for having me.

0:05:57.3 SC: So the standard model of particle physics, this is what people like you and I think about a lot these days. Let me ask like a silly question to start. Do you think that the standard model is beautiful or is it kind of an ugly duckling as theories go?

0:06:12.3 CC: Ooh, interesting question. I've never had it quite posed like that. I think in some ways, it is quite beautiful in the sense that there's a lot of patterns that we see. And in physics, really the thing that we love to see is patterns. So we see that there are three generations of a lot of things. We see that the gauge bosons kind of fit nicely in one bucket. We see that there's the pattern of things getting heavier and things coupling more strongly to the Higgs. So in that sense, it is a very beautiful model. But in the bigger sense of where did this thing come from? It's very ugly in the sense that there's not really a fundamental explanation as to why particles look and behave the way that they do.

0:06:56.3 SC: Well, let's just be nice to our historian listeners or whatever and explain what a gauge boson is and why you say they go into a bucket.

0:07:03.7 CC: Fair enough. Yeah. So in the standard model. There are basically two kinds of particles. They're either fermions or bosons. So the fermions are the things that tend to make up matter. Things like electrons are fermions, things like quarks, which are the particles that you find inside of nucleons are fermions. And then the gauge bosons are the particles that effectively tie everything together because they mediate the forces exchanged between these particles. So fermions often we like to think of as matter if they live long enough to survive, then they can be stable matter. And then the bosons are the force carriers. So things like electromagnetic fields, the particle description would be made out of bosons.

0:07:47.4 SC: And the standard model is, that's the label. It's such a boring label. It gives you the impression that it's just the model we had today and tomorrow we're going to change it. But there's a little bit more stability here.

0:08:00.1 CC: Yeah. That's kind of funny. Whenever I think of standard model, I kind of think of the most beautiful model, like the standard model, like the thing that you would think of when you consider a beautiful person. It's like, yeah, okay. So interesting that you have that. But I mean, yeah, so the model is inherently something that it's not supposed to be a first principles object. It models the things that we see, but it doesn't come from a deep a core principle. So in the sense that, yeah, it could move around if we find something that's in conflict with our current model. That's true. But so far, it's done an amazing job at actually accounting for a lot of the physical phenomena we've been able to see.

0:08:45.0 SC: Actually, that's something worth amplifying on, because maybe we don't always make it clear to folks, but the difference between a model and maybe a theory or a framework. We have quantum field theory as a very broad framework, and then this very specific thing of the standard model.

0:09:02.3 CC: Yeah, so this is a really subtle question that I feel like you kind of roll your eyes at the first time you're taught this distinction in school. But yeah, a model is something that we use to account for phenomena that we observe. So it's very empirical in nature. Like the standard model, we don't necessarily know why there are three generations of everything, but we observe them. So they go into the standard model versus something like grand unified theory would be a way of explaining sort of why everything comes together in the way they do. So certainly we can't talk about the details of grand unified theory if we're going to talk about other things. But yeah, the difference is, like you mentioned, that something is first principles to motivate where it comes from versus something that's a way to kind of categorize empirical evidence that we've seen.

0:09:50.4 SC: And the standard model of particle physics, as we call it, it does a pretty good job of accounting for what we see.

0:09:57.4 CC: That's a frustratingly good job, Sean.

0:10:00.9 SC: Tell us what that exactly means. Why is that frustrating and how good is the job?

0:10:05.6 CC: Yeah, so I mean, the kind of work that we do as particle theorists, is we want to basically stress test this standard model and either confirm very rare predictions that it can make. And so sort of see very hard to see phenomena show up in things like colliders, which is my personal specialty and realm of interest. And then also we want to see where it breaks. We want to see if there are pockets of predictions that are false or that are just lacking, or we see some phenomena that's unexplained by the standard model. And that's where it's fun to be a researcher, is when you try to come up with solutions to why this prediction isn't quite matching your expectation.

0:10:46.1 SC: It's a weird thing. We have a theory that fits so much data, and yet we're convinced it's not right. Why don't we just declare victory and do biology?

0:11:01.1 CC: Fair enough. And I think, in fact, a lot of people I've had that mentality. But there are some pretty big holes in particle physics. And I think kind of the phase transition that we've gone through as a field in the past few decades, certainly in my career, which has been, albeit not quite as prolific as yours, Sean.

0:11:21.0 SC: You're younger.

0:11:23.0 CC: But yeah, there's big open questions that are becoming more and more sort of nuanced versus like, ah, what the heck is this? Like that used to be the state of affairs is that we'd turn on our little bubble chambers and we'd look at something and be like, oh, what the heck is this? And that was a really rich, interesting time to be a physicist. And a lot of theories came and went. And we were able to make an amazing amount of progress in such a short period of time just from experimental evidence. And now I think we're in a much more subtle phase of particle physics where the questions are not so much what the heck is this, but where does this come from? Why does this look like this? What are the things that we're not seeing? Like things that are much more fundamental towards why has the universe taken on this profile?

0:12:16.4 SC: And I think that in certain corners of the Internet anyway.

0:12:21.6 CC: Oh, no.

0:12:22.1 SC: There's it's a big Internet out there. There's a lot of corners to it. But there are people who worry. They would almost give you the impression that the slowing down of discoveries in fundamental physics is somehow physicists' fault. You're not doing it right. And I try to explain, that's not really it. Do you have a favorite way of conceptualizing this?

0:12:43.2 CC: Yeah, it's so funny that you say it like that, because in some way, I feel a bit flattered to think that people think that my work is so important that I could, that anything consequential could be the fault of mine. It's like, well, thank you. My goodness. But I mean, yeah, it's just it's physics is a field that I think has always kind of suffered from being something that the public really enjoys as much as something like astronomy or biology, where people either see or experience much more of it. Because physics requires, as we are already seeing in this conversation you and I are having, physics requires so much lead up to understanding sort of why it's surprising or exciting or interesting or puzzling. So, I think that a lot of this rhetoric comes from the fact that the LHC turned on and we just saw the Higgs boson.

0:13:33.5 CC: And it's kind of a ridiculous sentence to me to say, oh, we just saw the Higgs boson because this was like the linchpin to making sure the standard model was even first order correct. So yeah, like the answer is just that there's not new particles showing up with sort of the unexpected frequency, which they were in sort of the 50s to 80s. And who ordered that was kind of the model particle physicists. But to say that that's a failure versus just the field has matured a lot, I think is underselling all the work that people have done for the past 70 years.

0:14:07.7 SC: But there still are looming questions. And you used maybe the perfect word here when you said they're subtle questions. It's not like here is a picture in our experiment that we can't explain. It's like we have a feeling there are deeper explanations and there are reasons to go look for them.

0:14:26.3 CC: Yeah, absolutely. So sort of the jargon that you might hear people use in particle physics is the hierarchy problem. And if you are not thinking about it all the time, it's kind of easy to write off as, well, is this really a problem? Are you just looking to keep yourselves relevant? But, effectively, how I like to describe it is, I loved all the sitcoms from the 90s or whatever, where it's like, oh, no, like something is going to close unless we find $35,629.15. And then behind them, you see the banner that's like, talent show, grand prize, $35,629.13. I think that was the same number. But yeah, like the fact that there are numbers that for some reason are so big and yet agree down to such a small accuracy is something that should be fundamentally puzzling.

0:15:22.9 SC: Good. So let's be a little bit more explicit. Now, we've stopped to the historians out there. Now, for the physics enthusiasts, what are the specific problems? You mentioned the hierarchy problem. That sounds like politics, not physics.

0:15:38.4 CC: These days, kind of hard to disentangle, honestly. Yeah, so the hierarchy problem is something specific to understanding the mass of the Higgs boson, which is one of the bosons in the standard model. And the Higgs is the weirdest particle in the standard model by far. It is the only particle that has the properties that it has. So like I said earlier, a lot of particles sit in three generations. The Higgs boson does not. The Higgs boson stands as a very weird outsider that you may have heard is responsible for giving particles mass. And if you want to learn all about that, there's a beautiful book by Matt Strassler that you should definitely check out.

0:16:16.1 SC: Very good.

0:16:17.1 CC: But the Higgs boson gives particles mass, has different what we'll call, yeah, intrinsic properties. It's called the spin and specific that doesn't match any other particle. And because of that, effectively, we think that the Higgs boson should have a mass 10 to the 18 times bigger than it does. And so this is the hierarchy problem. And the hierarchy is just the mass scale that we expect and the mass scale that we see everything else sitting at. And the fact that there's 10 to the 18 differences, really 10 to the 32, because it's squared and that's the real first principles number. The fact that something can be off by 32 orders of magnitude from our theoretical predictions, where did that come from?

0:17:05.0 SC: Right. But, okay, I'm going to be a little bit unfair to you. You use words like what the mass should be.

0:17:12.1 CC: Yeah, of course.

0:17:12.4 SC: And things like that. How do you know what the mass should be? What gives us that expectation?

0:17:18.0 CC: So this is definitely questions that when I was in high school. So I was in high school right when I was ending high school when the Higgs boson was discovered. So if you guys want to calculate how old that makes me, please don't. But, yeah, I remember having this exact same thought when people were like, oh, but is it the Higgs? Did we really discover the Higgs? And I was like, well, who cares? You discovered a particle and it's right where you thought the Higgs, why do you get to say, oh, is it the Higgs? And the truth is, like you said earlier, is that there are a lot of ways in which we can have actual predictions using frameworks and not just models.

0:17:52.1 CC: So the model is the thing that the mass that we observe is plugged into versus the theoretical framework is in how we make the predictions. So, given the properties that we hypothesized of the Higgs that make it a particle that could, in fact, give mass to other particles, then we have the theoretical tools to make a prediction of what the mass should be. And so basically, because the Higgs matched the properties that we modeled it to have, then we can actually use a theoretical framework to calculate what its mass should have been if things were as simple as we hypothesized to be.

0:18:30.4 CC: So the fact that there's something going on that we don't quite understand is one of the biggest open problems of particle physics to me.

0:18:37.0 SC: Okay, good. So that's the hierarchy problem. There's probably a bunch of other problems there. Dark matter is the obvious one. So you've thought about that.

0:18:45.7 CC: Dark matter is for sure, and I feel like it's kind of unfair for particle physicists to claim this a strictly a particle physics problem because it could be astro, it could be cosmology. Like there's a lot of different buckets in which you could put the dark matter problem. But certainly if dark matter has a particle description, that is also something that the standard model should try to sort out.

0:19:06.9 SC: And do you worry that much about neutrinos and their masses? Those are actual discoveries that we made over the last 25 years.

0:19:12.9 CC: [laughter] This is the biggest fight I've gotten to with my PhD advisor. Is neutrino mass new physics or not because one could argue that there exists a way to account for neutrino masses in the standard model. But my response to that is you can't do it with only fundamental interactions. You have to have something that happens a little bit more complicated. So because there does not exist a fundamental interaction term that we can write down for neutrinos that give them mass because they can't, it won't work the same way as the other particles with the Higgs boson. Because that that's not true. I think that is an example of new physics. However, I don't wanna start a fight on that today, [laughter], so maybe another day I'd be happy to start a fight but yeah, neutrino oscillations inherit violation of leptin number, something else is going on there. Right? Absolutely.

0:20:11.4 SC: Well, okay, let's indulge ourselves there a little bit and try to figure out if we can explain why you can't give neutrinos masses the same way you give electrons or quarks masses.

0:20:25.1 CC: Okay. How do I say this without talking about left-handed and right-handed fields [laughter]].

0:20:31.3 SC: Or talk about left-handed right-handed fields. Go for it.

0:20:34.8 CC: Okay. So what we've seen in the standard model is that there exists left-handed and right-handed fields. And this is sort of the fundamental difference between mass lists versus massive particles is the amount of what we call degrees of freedom, which you need to have an object sort of propagate. So for massive particles, they have a left and a right-handed component. And you can think about this, I think with sort of relativity is the best way to sort of understand it. So if you have something that's got Angular momentum let's say that it's turning to the right, if something is massive, then you as an observer can boost yourself either in front or behind of that object. And if you are in front, you see it turning one way, and if you're behind, you see it turning a different way. So you need to have an object that allows to spin both to the right and to the left to be able to describe that in nature.

0:21:33.5 CC: However, if the object is massless, then it is traveling at the speed of light and there does not exist a valid frame in which you can boost to flip that spin. So this is why you don't need both a left and a right-handed spinning degree of freedom to describe these particles. So in our standard model, we have left and right-handed degrees of freedom for all of the quarks and the electron and the muon, which is my favorite particle and the tau. But we do not have that for the neutrinos. For the neutrinos. We only have left-handed field components. So if they were to get mass, you would need to have either two neutrinos and two Higgses interacting, which is different from the fermions, which is just one Higgs and two of the fermions. But that presents, yeah, that's something that we have not been able to verify if this is right. So neutrinos could be what we either call majorana fermions where they don't have to have this left and right-handed story, or they can be Dirac where they do have a left and right-handed story and we just haven't found the right-handed component of the neutrino yet.

0:22:46.2 SC: Good. So.

0:22:46.4 CC: I don't know if that was way too technical or not [laughter]

0:22:48.8 SC: No, I think, well it was goodly technical. I like it, but maybe we can boil it down. So in the standard model for the other particles, for the other fermions, we know they have a right-handed part and left-handed part. And then the Higgs sort of glues them together and gives them mass. For the neutrino, we know they have a left-handed part. And the right-handed part is not necessarily there, but you need it to make mass or you can be more tricky.

0:23:17.8 CC: Yeah. So if you want it to basically have the same sort of mechanism as the other fermions in the standard model, which would make sense because again, as physicists, the thing that we say is beautiful is symmetry. If you want it to have a left-handed and right-handed component, then we just need to find a new particle that is the right-handed neutrino, and then it could get its massed through the Higgs and things could be similar but a bit different to the rest of the standard model. The other option that I said before to do perhaps what's, majorana fermion instead is that you don't need the left and the right-handed is that the neutrino. You just need the one degree of freedom in the neutrino. And then if you have two of those degrees of freedom put together, stitched together with two Higgses, then you can also have the same sort of mass giving mechanism. But again, that would look very different from the standard model. So the fact that all the particles get their mass one way except for this one kind of particle, which gets their mass a different way, that's still a very interesting question. And to understand how that came to be is something that would require further study.

0:24:22.2 SC: Right. Okay. So very good. So along with the hierarchy problem, neutrino masses, we can come up with theories that explain them, but they're pointing toward things that we haven't yet found. This is what motivates people like you to continue on the search for new particle physics.

0:24:37.1 CC: Yeah. And we haven't found definitive evidence that a single fundamental interaction could explain this. And I think as particle physicists, we love when there is a functional description, which again is a standard model, but to have a fundamental description, I think that's really what we all chase.

0:24:56.2 SC: Good. Very good. And the last thing I wanted to mention were muons. You've already mentioned them. I'm not sure this counts in your mind, but the Muon is basically the heavier cousin of the electron, and then there's the tau, which is the heavier cousin of that. And one can ask why are there three copies of all these particles? Is that one of the puzzles we worry about?

0:25:20.4 CC: Oh, absolutely. Yeah. So this puzzle comes in a bunch of different names. I think to kind of put all of them into sort of one area that we could describe it, it's kind of the question of flavor physics. So I don't suspect particles have that different of taste, but what do I know, [laughter]? So flavor is just sort of another name that we give to particles to describe how properties are different. So charge is the one that we all learn in school and that we're most familiar with because there's plus and minus charge, but particles have a lot of properties and we just kind of need names for them. So flavor is the name that we tend to give the different generations of particles. So muons are a different flavor than electrons. Again, how they taste is not something I can comment on [laughter], but yeah, so we call this sort of the flavor, flavor physics is sort of the study of understanding why the different generations behave a bit differently.

0:26:15.7 SC: And so what's the answer to all these questions? I mean, I know that super symmetry was out there for...

0:26:20.8 CC: Sean if I knew [laughter]

0:26:22.2 SC: You would reveal it on this podcast. I'm pretty sure.

0:26:24.3 CC: I would and the Nobel Prizes would come showering upon me.

0:26:27.9 SC: And I would be in your acceptance speech, thanking me for...

0:26:32.7 CC: You would. I think we'd have to share it honestly.

0:26:33.9 SC: But Okay. For a long time the particle physics community was very excited about super symmetry. And they were hoping to see it at The Large Hadron Collider, et cetera. Maybe enthusiasm has cooled for that, but not completely gone away. What is your take?

0:26:50.4 CC: Yeah, so super symmetry is a pretty well named thing in particle physics. It's like taking the symmetries that we have, but then more so it's super symmetry and basically it's adding one extra symmetry into sort of our description of space time itself. And then the consequences of that tend to be... In the simplest description of it is that all the particles we've seen in the standard model have what we call very acutely super partners. And basically they're the same particle with very similar properties except for fermions become bosons and bosons become fermions. So super symmetry was an amazing idea for a lot of reasons. One, because it introduced an extra symmetry, which again, we all love. And if there's a way for a symmetry to exist, oh boy, do we want it to exist.

0:27:43.1 CC: And the problem that it really famously addressed is exactly this hierarchy problem is that if you want to understand why you have a very, very big number as a prediction, but you see a very, very small number experimentally measured, the easiest answer is there's a symmetry that cancels something. Right? A symmetry is a fancy way of just saying that there's basically a copy of something. So the symmetry is a way of explaining why two big numbers should almost exactly cancel. So super symmetry as we could have seen it before, The Large Hadron Collider turned on would've been an amazing way of explaining why the Higgs boson has a mass of around a hundred GeV instead of 10 to the 18 GeV. So that was kind of the most exciting promise is that there was a fundamental reason why this particle was so light and there was expectation of all these new particles that we would hope to see.

0:28:41.2 CC: And it was going to be an amazing time and people were even worried that we couldn't find the Higgs-Boson because there'd be too many of these other super partners. And unfortunately we turned on the LHC and we did not see the super partners. So super symmetry as theorized in its most beautiful, pure form of having the maximal symmetry is not something that's probably realizable at this point. However, there are versions of it in which you can sort of introduce new particles or new interactions that sort of take you away from that perfectly symmetric case and sort of break the symmetry. So of course these theories still exist and it's still worth looking for, assuming that we have the tools to do so, but at some point you're not solving the fundamental question that you asked, or you have to introduce something that basically replaces the fundamental question that you were asking. So it becomes a bit of a patchwork solution rather than a global solution. And that's something much less attractive.

0:29:38.8 SC: So if we were having this conversation 20 years ago, you might have been very excited about your super symmetric grand unified theory that was going to predict, that was gonna solve the hierarchy problem and give you the right dark matter and explain neutrino masses and evidence for it would be existing at the LHC and that did not happen.

0:30:01.2 CC: Yeah, I mean, it is definitely something that really is a marker of a very healthy theory in physics, I think is when it can sort of address many problems at once versus just picking one problem and trying to, like I said, patchwork it. So yeah, it was a really beautiful theory that had a lot of reasons to be motivated, could address a lot of questions that we had about the standard model. And yeah, the fact that we didn't see it, I think has put us into a little bit of a crisis in terms of the theory world and the particle physics community.

0:30:34.3 SC: But it is, you can see why it's a little frustrating. I mean, the puzzles that you've talked about sound like to me a very good motivation for the need for new physics out there. And we could have found it all at The Large Hadron Collider.

0:30:47.6 CC: We could have had it all.

0:30:49.2 SC: And we didn't. And it's not that we disproved the theories, right? I mean, super symmetry could still be right or whatever, it's just that they're hiding from us.

0:31:00.0 CC: Right. Yeah. And I mean, I think in particle physics certainly if you look back at the history, there's been a bit more of a give and take between theory and experiment. And so we were functioning for a long time before The Large Hadron Collider came on. We had the Tevatron at Fermilab, which did make important discoveries too. But really going up to that sort of energy frontier that we could have with the LHC was so important for the field. And we were really driven by theory for a long time and we had this beautiful promise that there was going to be something at a hundred GeV, right?

0:31:40.3 CC: We had many predictions that were just fundamentally breaking down that told us there had to be something up there and we hoped it was the Higgs, but it could have been other things too. But we knew that there was something up there and now we just don't have that theoretical promise, right? Is that we just know things are broken. And we haven't yet been able to debug the standard model. So yeah, to me it kind of feels like now it's the time to let experiment drive a little bit and see what's up there and maybe as theorists we can look at data and get inspired again for what might be a good solution.

0:32:11.5 SC: Is there still room for The Large Hadron Collider itself to discover new things?

0:32:18.7 CC: I am of the opinion that yes The Large Hadron Collider is definitely still a machine that has some discovery potential. I think we have this kind of picture, certainly people who know a little and not a lot about Collider physics have an idea that The Large Hadron Collider just turns on and then it's like this Boolean output, like new physics, no new physics, and there's just so many subtleties that occur between colliding the particles and a physicist understanding what's going on, right? So the way that we choose what events to look at, the way that we analyze the events, the way that we interpret the events, like there's so many things in which you could introduce a bias that would skew you away from understanding fundamentally what physics could be going on. I don't think that it's probable that we'll discover something new at the LHC, but is the question, could there be hints of something new absolutely?

0:33:19.8 SC: And it's even possible following what you sort of alluded to, that the LHC has discovered something new, but we haven't quite analyzed it in the right way to notice.

0:33:30.5 CC: Yeah. And this is kind of what I did my PhD on in fact, is the idea of how we can sort of robustly look for new physics effects. Because again, we're likely not gonna get, at this point with the LHC, we're not just gonna see some beautiful new resonance just falling out at a perfect sharp peak at like 2 GeV. It's possible, but it's probably pretty unlikely. So you kind of have to use more fundamental theoretical tools to say where are inconsistencies possible to show up versus let me just wait for the most beautiful evidence of new physics to fall into my lap.

0:34:04.2 SC: Right. Good. Well that's good. This is gonna keep you employed for a little while. That's nice.

0:34:08.2 CC: That's what I hope [laughter]

0:34:10.9 SC: So let's allow ourselves then to be starry-eyed and optimistic, and imagine we're gonna build new particle accelerators to go beyond what the LHC does. Maybe to soften us up, could you explain sort of the fundamental difference between colliding protons and other hadrons together versus colliding electrons and other leptons together?

0:34:32.0 CC: Yeah. So this is a great question. And every time certainly as a PhD student, when I listen to it, I would hear someone describe one machine and be like, well this is the best collider. And then I would hear someone describe the other one. I'm like, no this is the best collider. And the answer is they both have strengths. So when you collide something like an electron, you're basically just colliding electrons. It's a fundamental particle. So electron plus electron combines. Usually we do particles and antiparticles. So you have E plus E minus come in, collide produce a charged neutral state. And all of the energy of these two electrons colliding can be recombined into different massive particles, different momentum particles, as long as the net energy and momentum of the event is conserved. However for proton protons, protons are actually a big bag of stuff.

0:35:23.8 CC: And that bag includes the three quarks that usually we talk about when you first take your nuclear physics course or whatever, in school, right? I don't know if many schools have nuclear physics, but chemistry, let's say chemistry. You have up and down quarks basically are the primary constituents of protons at low energies. But these quarks are tied together with gluons and inside the quarks or sorry, inside the protons, especially as you start cranking these things up to really high energies is the bag becomes much more complicated. And inside these protons, we have particles that are very cleverly named as partons because they are parts and particles have to end in on. So, Partons.

0:36:06.6 SC: There you go.

0:36:08.7 CC: Yes.

0:36:09.2 SC: I think that's Feynman fault, right?

0:36:10.3 CC: [laughter] Well he did enough good things, so we can give him a break for this one. But inside this proton, all of these different partons, which include the gluons, which are those bosons that we talked about earlier, and the quarks, they kind of share the total energy of the proton. So at the LHC, we collide protons of seven ish, TeV. And no one particle inside that proton is going to have anywhere near seven TeV. Tends to happen that the gluons take most of the energy, and then quarks also take some of the energy.

0:36:45.1 CC: So when you're really looking at a collision, it's a gluon gluon collision or quark, quark collision, or even in the proton, sometimes you can have quark, anti quark pairs pop into the vacuum or pop out of the vacuum and then disappear again. And sometimes you can collide those so you can have a quark, anti quark interaction. And all of these things will share the energy of the proton. So in some ways that makes for a super interesting collider because the opportunities of what kind of particles you can collide is much bigger, right? You can collide not only up and down quarks, but strange quarks or charm quarks or gluons. And some things more rare than others, of course. But it means that you can see all sorts of interesting signatures come out.

0:37:27.0 CC: The downside is that you never know exactly what the energy of these things are. So that can make your analysis much, much harder. And of course, you don't get that full energy of the protons. So even though the LHC runs at 14 TeV, we don't actually get to see any collision happen at 14 TeV. It's usually much closer to one or two.

0:37:47.7 SC: Whereas if we collide electrons together, we can know exactly how much energy is going into them.

0:37:54.3 CC: Yeah, often you'll hear these two machines sort of described as electrons are the precision machine. Because everything's very clean, right? Like you just have electron in, electron in. And then you know the energy and the collision can be basically completely reconstructed, assuming what comes out. Versus for protons, it's like smashing cars together. There's debris everywhere. You don't know exactly what collided into what. And it's kind of like hitting something with a big hammer and just a bunch of stuff can come out. But knowing exactly where it came from and how it came to be is a much harder question. So protons are called... Yeah, sorry. Electron-electron is precision. And then proton-proton is often called discovery because you can have all that high energy available to you.

0:38:37.2 SC: It would seem that we're in a discovery kind of mood right now. I mean, precision sounds like it's good for studying things we've already discovered. But now we would like to discover some new particles, no?

0:38:48.1 CC: Yeah, so that's a great point. And maybe the way that these things are named is kind of unfair to 𝑒 + ⁢ 𝑒 − machines. So like we were talking about in the beginning with the standard model. Part of understanding the standard model is knowing to what degree our predictions are correct, right? Because in science, you never get to say definitively, "Oh, this number is correct." You can only measure it to a certain precision. And that's sort of the claim that you can make. So with precision machines, precision machines are standard model machines in the sense that they try to measure standard model things. But they are also discovery machines in sort of a roundabout way in the sense that if you were to discover something that is not matching the standard model prediction, that's a hint of a discovery, right? So you don't get to actually physically make the particle and point to it and say, "Look at this, we did it." But you get to say, "OK, there's a discrepancy in our data. And this could be indicative of new physics." So I guess that's why we call it precision versus discovery because it can't make, it can't concretely define unambiguously that there is something new going on. But it can absolutely help us sort of know where to look when we go to the higher energies.

0:39:57.1 SC: So in other words, there's so many predictions made by the standard model of particle physics that you can test them all. And if any one of them is discrepant, you determine that there must be new physics, even though maybe you don't know what it is. But then the theorists will have a field day writing papers about what it could be.

0:40:14.4 CC: Yeah, absolutely. And even if something's not new physics in the sense that there's a new particle or a new degree of freedom that we haven't accounted for, the fact that there could still be new phenomena that we haven't understood is still, of course, a super exciting discovery to make.

0:40:29.2 SC: And are there plans or at least sort of ideas on the drawing board for building either higher energy proton colliders or electron colliders?

0:40:39.7 CC: Oh, boy. So this is the question of the decade for collider physicists. So there are a couple of different ideas that people want to get into. So we can go as slow or as fast through this part as you want. But to summarize quickly, there are basically three kinds of machines people want to think about making. One is a linear 𝑒 + ⁢ 𝑒 − collider. So you collide electron-positron or electron-anti-electron at reasonably high energies, so around a TeV or so, or maybe just near the Higgs mass to make a bunch of Higgs bosons. But you collide them in a line. So you don't get to circulate. You just collide them in a line. They either collide or they don't. And that's the end. Another option is to use circular colliders. So this is what the LHC and the Tevatron were is that you circulate these particles. So if they miss their collision, they still have another chance. And if you ever studied electromagnetism in school, you know that putting things in a circle is very different than putting things in a line. So that comes with its own complications and we can certainly get into that. But in terms of circular colliders, we either think about doing a lepton collider. So either electrons or my favorite muons, which are certainly an immature technology, or doing protons. So basically doing a bigger, badder version of what we can do at the LHC.

0:42:06.3 SC: If you were a betting person, putting aside the muon collider...

0:42:12.1 CC: Which I am.

0:42:13.6 SC: The muon collider we're going to get to, that's the payoff here. But what is the leading candidate for building the next collider other than the muon collider?

0:42:27.9 CC: So since these are pretty big scale projects and since we are kind of in this exploratory range of particle physics, I think that the attitude that a lot of these funding agencies and lab directors and experimentalists who actually want to see things happen versus just be like me and dream with a Mathematica notebook. I think given this climate and the fact that we can actually study a lot of Higgs physics with somewhat low energy things, I think the current attitude is to focus on lower energy circular electron colliders and sort of the two places in the world that are presenting the most on-shell concepts of these projects would be China with the Circular Electron-Positron Collider and CERN with the Future Circular Collider or FCC and then the FCC-ee, so Electron-Electron Collider. But of course, one day it might not be future, so we'll have to rename it, but that's what it is for now.

0:43:24.9 SC: I love how you, Cari, you use the idea of being on-shell as just a common adjective that people would know.

0:43:32.5 CC: I mean, I talk to a lot of physicists, Sean.

0:43:37.3 SC: Yeah. Okay, I noticed that the United States is not included in there. Have we basically dropped out?

0:43:41.5 CC: So for this next round of experiments, the thing that the US has chosen to invest in right now is neutrino physics. So Fermilab, which is sort of our flagship particle physics laboratory near Chicago, Illinois, is committed to doing a big experiment called DUNE, and that's sort of measuring neutrino properties. So the lab will have to go through a lot of updates in order to make this experiment the most efficient version that it can be. And that leads us for a lot of possibilities for doing things like research and development at Fermilab. But I think that in combination with the fact that other big laboratories in the world are willing to and have put a lot of time and effort into sort of making a more concrete plan, yeah, the US is not going to be likely where we have the next 𝑒 + ⁢ 𝑒 − circular collider.

0:44:37.3 SC: And just so for sort of cultural enrichment purposes, Fermilab, which was the home of the Tevatron, which was for a long time before the LHC came on, the highest energy particle accelerator, out there. The Tevatron is now just shut down. It's in mothballs, right? They don't keep running it, even though the next thing is turned on.

0:44:57.4 CC: Yeah, that's right. So I remember I actually grew up somewhat near Batavia, where Fermilab is. So when I was in high school and I was just a fan of physics, I had this great T-shirt that was like the Tevatron, like 10 years running. And then like the next year they announced like, "Well, we're shutting it down for the LHC." And I was so I was so heartbroken. I'm like, why would they turn it off? But yeah, these things are not cheap to run. And the fact that some other experiment could be doing basically its physics program more efficiently and then also more means that, yeah, it's probably not great to have too much repetition for these kinds of experiments. The Tevatron did a lot for particle physics.

0:45:37.3 SC: Sure.

0:45:38.8 CC: But now that the LHC had turned on, it just makes sense to sort of let it carry the torch.

0:45:45.3 SC: Okay, and then we have the possibility of a collider using muons, which is kind of a compromise. Like muons are heavier than electrons, but they're simple, unlike protons. So is that a good way to go?

0:45:58.3 CC: Yeah, so this is to me what I think will be the future of particle physics. So an 𝑒 + ⁢ 𝑒 − machine is very safe in the sense that we basically know how to build it. And we think that we have all the technology that we already would need to be able to make it work the way that we need it to work. A muon collider is a big risk, big payoff kind of machine. And of course, as a theorist, I get to just say, Ah, of course we should invest in this because my whole life is dreaming, Sean.

0:46:30.9 SC: Easy for you to say, yes.

0:46:34.7 CC: But yeah, a muon collider is a really exciting new option because like you said, it sort of combines the aspects of being both a precision machine because they are fundamental clean objects. You're not colliding bags of stuff. You're combining, colliding two individual particles. And because a muon is heavier, we can accelerate it to much higher energies than electrons. So an electron circular collider really can't surpass more than a couple hundred GeV, even going up to more than 300 GeV for electron-electron is a big ask. And knowing if we have the magnet technology and even just the power to be able to do that is not clear at this moment. So with a muon collider is that you can break that frontier of higher energies than we've ever been able to go. You can do it in a circular machine and you can do it in a somewhat clean environment, given the fact that muons are fundamental particles. So when you hear this kind of stuff, I don't know how you can not be excited, right? It's such a beautiful promise of everything you could want put into one collider.

0:47:37.6 SC: Well, that's 'cause you have not yet told the audience that the typical muon decays in about two milliseconds.

0:47:42.7 CC: Yeah, well, that's kind of a bummer, right?

0:47:44.4 SC: Microseconds, sorry, microseconds.

0:47:44.8 CC: Yeah, so the thing with these higher generation particles is that because they have all the same properties as the lower generation particles, if you want to be fancy, we'll call them quantum numbers. And they have higher mass is that they have this really unpleasant tendency to want to decay, which is why atoms are made out of electrons and not muons, because muons, like you say, live for 10 to the minus six seconds and then they decay away to neutrinos and electrons. So one of the most fundamental challenges that we would have to overcome if we were to make this amazing new machine would be to accelerate particles that decay, which we have never even tried to do on this kind of scale before.

0:48:32.4 SC: So you have a millionth of a second to make a muon, to hopefully make more than one muon.

0:48:36.8 CC: Hopefully.

0:48:40.2 SC: Make a whole bunch of them, gather them up and accelerate them around a ring that is kind of big and then collide them together. That's the challenge.

0:48:46.2 CC: Yeah, and when you say it like that, not so bad, eh?

0:48:50.5 SC: Well, it's...

0:48:51.7 CC: Just be quick, it's fine.

0:48:51.7 SC: Make no small plans, as a famous Chicagoan once said. How do you make all these muons? Let's get in the nitty gritty of pretending we're experimentalists. We can do that because we don't need to worry about all the details, but...

0:49:09.6 CC: That's right. Yeah, so this is already hard. So what we would need to do to make all the muons, we produce them as tertiary particles. So usually what would happen is if we have protons...

0:49:21.6 SC: That sounds scary already, yeah.

0:49:21.7 CC: I know. With protons or electrons, right, basically you just ionize things and there's all your particles. They're stable, they're abundant. But with muons, what we do first is we need to accelerate protons to pretty low energies, so order of a couple GeV. And again, for reference, at the LHC we collide things at TeV, so a thousand times slower than what we do at the main collider, but still with some acceleration technology. So we accelerate protons to a couple of GeV and then we just dump it into some chunk of metal. So sometimes lead, sometimes tungsten. The chunk of metal that we dump into is in fact a very sophisticated field of research, so my apologies to people who work on targetry. But you dump your protons into this material and then they scatter around and they produce mesons. So mesons are even simpler in some ways than protons. Protons are baryons 'cause they have three quarks. Mesons have a quark and an anti-quark.

0:50:24.7 CC: And they're lighter than baryons most of the time because they're made of less stuff. So these mesons can be produced and because they're lighter than the proton, that's usually what they'll be producing in the scattering process and you make a ton of mesons in this process. And then the mesons are also unstable and then they want to decay. So a big way that mesons tend to decay is into muons. Almost primarily they always decay into muons and a muon neutrino. And so from there you have this big cloud of muons that are being produced by this target. But because your protons are slow, particles aren't boosted forward and because your mesons are even slower than your protons, they're also not very forward. So you have this really huge cloud of muons that are not at all bunched together in the pin-tight way that we would need to collide them. So that's just the first step. And after this already extremely difficult to engineer and optimize step, you have something that couldn't even begin to be accelerated. But you have your muons at least.

0:51:28.1 SC: And they're all negatively charged. So not only are they in a puffy cloud, but they're repelling each other.

0:51:36.6 CC: Yeah, yeah. So this is something that we do need to worry about because of how tight we need that muon bunch to be.

0:51:46.4 SC: Basically because you want to collide it into another bunch. And if they're all big and puffy, that's never going to happen.

0:51:49.0 CC: Exactly, right. It's like colliding a few, like BB pellets, right? It's like the further away they get, the more diffuse they are. And the fact that you might collide one BB pellet with another after they travel some non-trivial distances getting to be even smaller probability than what's reasonable to expect. So yeah, when you produce the muons like this, of course, you get both mu plus and mu minus and you scoop them off until there are two different ways to process them. But yeah, once you try to crunch 10 to the 13 muons into a cubic millimeter of space, things start getting a bit tricky.

0:52:28.4 SC: So is this something that we have the technology to do? Or do we have ideas to do it? Is this an ongoing research program?

0:52:35.4 CC: Yeah, so this is basically the biggest open question that we need to resolve as a muon collider collaboration. And so this is called 6D cooling, which sounds very cool. It sounds like you're in higher dimensional space. And really what it means is the sixth dimension is three dimensions for momentum and three dimensions for physical space, because you need all these muons to be traveling not only at the same momentum, but also localized to a very small bunch. And so accelerator physicists have been working on this really for 30 years. And they've made a huge amount of progress, certainly in the last 10 years. But basically what they do is they design these different ways that you have to have this process of basically taking momentum from the muons because you can't just squeeze them together. It's like squeezing one of those plastic dog toys, right? You squeeze it in one direction, it explodes in another direction. So you need to lose momentum from the system. And then you can use magnets crunch it back together. So it's this constant process of take momentum, give momentum, take momentum, give momentum. And they need to do that basically a hundred and some times before muon even decays. And then you need to accelerate it. So this is just the process of getting it ready to accelerate.

0:53:55.8 SC: And you have a millionth of a second.

0:53:58.5 CC: And you have a millionth of a second.

0:54:00.3 SC: And then, okay, we're probably pretty good at the actual accelerating. I mean, that's something that we have been doing for a while.

0:54:07.6 CC: You would think, but even that's hard. So accelerating particles that decay again are an entirely new challenge.

0:54:17.6 SC: Okay.

0:54:17.7 CC: Because one, everything in your detector is being constantly sprayed by the decay products. So there's all these other robust things that you need to account for when you're designing this. And the fact that we don't have the time effectively to do the same kind of acceleration that we do at the LHC. At the LHC, we just kind of ramp things up and it takes 15 minutes to get those protons up to the speed. We don't have 15 minutes.

0:54:44.9 SC: We don't have that time, no.

0:54:46.6 CC: We have a microsecond. So the kind of magnetic field that you need to set up to make this feasible is also extremely different.

0:54:54.3 SC: And I remember, this might literally have been before you were born, but I remember Chris Quigg talking about a muon collider many years ago. Chris Quigg, former Mindscape guest, as well as a physicist. And there was something called the ring of death because the muons, once they're in the circle, keep decaying and giving off neutrinos and other particles, which then go off and kill all the cows in the field.

0:55:24.3 CC: Oh my God. I love how dramatic we are sometimes.

0:55:27.0 SC: The particle accelerator. Is this something that I should be worried about? Or is that more or less something we have under control?

0:55:31.5 CC: All right. Well, thanks Chris for saying that. Geez. Yeah, it really cracks me up because neutrinos, when you talk about them in physics, most of the time, it's just kind of like, "Who cares? Who cares?" Neutrinos. They're not going to get near a detector. Don't worry about them.

0:55:47.8 SC: They'll go right through you.

0:55:49.0 CC: But now that you can attach the word death ray or death circle to them, people are like, oh my God. Neutrinos? So yeah, the physics behind it is that yes, neutrinos that are more energetic will wanna interact more. And so we haven't had to worry about this in the past 'cause we've never been producing TeV neutrinos in a large quantity. The worry that we would have is not that neutrino raised Zappa cow and then suddenly it's raining steak. That's not quite the picture. But what can happen is that all of these neutrinos that are coming off from your muon being circulated is they'll just travel in a straight line. They'll escape the experiment and they'll travel through dirt, right? Like they can just go through dirt and if there are high enough energy, they might interact with some atom in the dirt and they could excite the atom, and then that could decay.

0:56:44.9 CC: So it's the radioactivity of neutrinos activating atoms in the ground. So if this stuff is either sufficiently underground or we have ways of absorbing the neutrinos before they permeate too far, or something, again, this is accelerators. Accelerator physicists are really, really great people that think of all kinds of wacky stuff that I would never have thought was reasonable. But you can... And this is the scientific term. You can wiggle the beam, and then when you do that, it's a diffused enough beam that you're not activating any one patch of ground too much. And so the overall radiation dosage, I hate to be the person to tell you this if you don't know it, but you're always being irradiated. There are always things irradiating you. You just need a sufficiently small dose to not notice. So if you can do that, then it is in fact well below any sort of legal limit and dangerous limit that you might be approaching.

0:57:41.5 SC: There might be a public relations problem here if you say, don't worry, we can wiggle the neutrinos, so the deadly dose of radiation is spread out over a wider area.

0:57:47.7 CC: Yeah. Just wiggle it.

0:57:52.0 SC: Yeah. Okay. I'm gonna...

0:57:52.8 CC: The wiggles.

0:57:52.9 SC: I'm gonna trust that OSHA is on top of this. But, all right, so I wanted to get the challenges on board, and I think we've done a good job with that unless there's any other secret challenges that we need to.

0:58:03.7 CC: Yeah, there's... Failure is possible at every step. And I recently, I actually just got back from Fermilab yesterday to attend this really interesting workshop where people sort of get together and we're talking about what we need to make this happen. And someone showed this really beautiful table of basically all the ways in which we could fail, and what impact that would have on the net collisions. And so yeah, the steps of failure are, first we don't produce enough muons, so this targetry thing doesn't work. We melt the target by just dumping constant protons on it. That could fail. Cooling it could fail. We might not be able to, in fact, cool it quick enough to actually have a sufficiently high, a dense enough muon beam to get any sort of reasonable number of collisions out.

0:58:53.8 CC: So the cooling is by far the thing that could take us out the most. And that's the thing that we need to prove, works before we actually make any sort of big steps to making the full scale collider. So when people say muon collider R&D, basically we're talking about showing that this cooling and acceleration can happen, right? The cooling or the acceleration around the actual ring is hard 'cause again, that requires an entirely new mechanism for accelerating things quickly. And then even just reconstructing the collisions is hard, 'cause again, you have all these decay products shooting off of it. And how do you make sure that your detector isn't constantly overwhelmed by the electrons and neutrinos coming out of these decaying muons? How do you actually see the physics of the collisions and not just the physics of muons decaying? So that's also hard. And then of course, yeah, neutrino death beam, let's stay away from that phrasing. The neutrino radiation can be mitigated with wiggles.

0:59:49.8 SC: Well, and maybe again, just for cultural enrichment purposes for the audience, there's a kind of physicist whose job it will be to figure out, okay, how do we build a detector that can distinguish the actual muon collisions giving us new physics from sort of the background radiation and things like that? And then there's another kind of physicist who's in there with a soldering iron and building the machine. And there's another kind of physicist, I think like you, who is saying, okay, here's a model that makes a prediction that we could actually test in a machine like this.

1:00:21.9 CC: Yeah. And there's also even a whole bunch of physicists that figure out how the magnets will bend the muons. So like, really every sort of difficult problem we have, there's an entire specialty dedicated towards solving the problem. That being said, we definitely are a bit human power limited at this point. So if you're out there and you say, man, the thing I would love in life the most is to accelerate muons. Oh my God, please call us.

1:00:50.3 SC: Well, it will take a while to do it so we can get the youngsters excited about this, right?

1:00:53.6 CC: Yeah. If you are 10 and you wanna be a part of the most exciting human experiment ever made, please...

1:01:02.1 SC: There you go.

1:01:02.7 CC: Stay in school.

1:01:02.8 CC: We need you.

1:01:05.8 SC: And okay. But let's switch to why this is worth all of the hassle. What's so great about a muon collider?

1:01:12.5 CC: Yeah, so I think there's a lot of ways to answer this question, and I think all of them are kind of equally valid. So I think the most obvious answer that you can have is, we are scientists and we wanna know if you can do something, right? So this is a kind of collider that we have theorized, and there's no fundamental showstopper that would suggest that this is a deeply impossible task to do. So the fact that this is just a scientific challenge to see if you can collide muons, and this could open up the entire, like an entire new generation of colliders, which are really effectively microscopes, right? For the fundamental interactions. The fact that this is a possibility, to me is worth doing, right? Like, if you can really open up an entirely new way to do a physics experiment, that's awesome. That's just... That in itself is very cool.

1:02:08.5 CC: In terms of understanding sort of the fundamental problems that we talked about in the beginning, again, what's exciting about Muon colliders is this was kind of if the technology can work, which I understand is a preposterously big asterisk to put on all of this, but if the technology can work, this is by far the fastest way to take us, the fastest and the most energy-efficient and compact way to take us to that energy frontier. So given that we are kind of stumbling in the dark right now in terms of understanding where the Higgs comes from, what dark matter is, do neutrinos get mass from the same mechanism or a different one? Is there anything new about the standard model? Are there fourth generation particles? That one's a little bit wacky, but the fact that there are so many open questions and no one beautiful theory like we had for super symmetry sort of explains them all, to me means that we should just be explorers. And we should be kind of agnostic and just do experiments in which we can sort of get the most return on our money in terms of just seeing new things. So going to that energy frontier, having a clean environment, getting high luminosity, that to me is how we're gonna get unstuck in particle physics. So muon colliders being the fastest way, the cleanest way in a really novel way to see things that we've never seen before, makes it something that I feel like we have to invest.

1:03:27.5 SC: And you used two words in there that sort of sound prosaic, but are super important, which is compact and energy-efficient, maybe that's the words. But compact, who cares how much space it takes? Don't you have space and energy efficient...

1:03:42.3 CC: Oh my God.

1:03:43.1 SC: Like, can you just plug it in? Why is that a big constraint?

1:03:46.0 CC: I know. I think so much of physics, there's like the meme that they have, the midwit meme, right? Where it's like the bell curve, and then you have like someone saying something stupid on one side, and then someone saying something a little bit educated in the middle and someone saying something, the same stupid thing, but with a lot of wisdom. And that to me is physics in a nutshell. It's like when you first start physics, you're like, oh my God, the detectors underground. Like what if the ground shakes? And then you're like, no, come on, people have gyroscopes and there's so much engineering and the machine is so heavy. And then you're like, oh, no, but the ground shaking does in fact cause problems. And this to me is the same thing with like yeah, space use and energy efficiency. At Fermilab, they're literally trying to plan what energy they can reach at a muon collider given the amount of space we have at Fermilab. 'cause if you don't own the space, you can't put a collider there, it turns out. If you are okay with us tunneling into your basement and you live in Batavia, again, please call.

1:04:50.6 CC: But yeah, like you just, if you run into mountains, you run into underground rivers, you run into state borders, and so you just can't build an arbitrarily big machine, expect that yeah, you as a scientist are immune to property law. So that's something silly. And also just the cost of building a tunnel. Tunnels are so expensive. Oh my goodness. So the smaller you can get it, the cheaper it can be. And again, it's not... We don't live in a fantasy world where every scientific thing worth doing is funded. So if you could have something with a smaller tunnel that gives you a whole bunch of physics results, that's probably the way to go. And with energy, I don't know how political we get on your podcast.

1:05:34.2 SC: As political as you wanna get.

1:05:37.5 CC: The whole Russia-Ukraine war, that is zapping Europe of all of its energy and the LHC is even having reduced run capacity, 'cause there's just less energy to be had in Europe. And quite frankly, if you need to choose between warming people's houses and colliding protons, I understand that tough sacrifice we have to make. So yeah, we live in a world where there's global warming and there's war and there's territorial issues. And the fact that we are scientists doesn't put us above any of these issues, and we still have to be responsible stewards of public money in space.

1:06:16.5 SC: Yeah. That makes a perfectly good argument. And I noticed you didn't say that you were actually in favor of shutting down or turning the LHC, giving it less power and to help the Ukrainians, but you understood why people would be, which is, Yeah, that's good. That's how physicists have to think, right? 'cause no one else is gonna do it for us.

1:06:34.9 CC: Look, Germans use a blanket.

1:06:36.3 SC: Yeah. Exactly. So you mentioned Fermilab there. Does this mean that the US... I don't really honestly care in some sense where it's built Australia, India, these are all fine places, but is the US one of the places that is contemplating building muon collider?

1:06:54.5 CC: Yeah, so I think this is also something very exciting to me. On a personal note, again, since I grew up in Illinois and definitely my parents were not physicists, but science supporters generically is having Fermilab close to your house just gives you an appreciation that things are dynamic and it really... It makes you feel much more energetic about the idea that, oh, I could do research or that research is happening and it definitely gives you a sense of excitement about it all. So I also have this kind of agnostic approach that, oh, I don't really care where it happens, I just want it to happen. But when you're close to it really, you feel it. You really feel it like, oh wow. There's an energy in the air that's not neutrinos and you get excited and it's just cool to be a part of it.

1:07:51.4 CC: So this is something that's really exciting as someone who's currently doing science in the US is the fact that the US might wanna re-enter the world stage of particle colliders is amazing. And I think this is very indicative, and this might be the spiciest thing I say. So I think the styles of doing physics in the US and other places, Europe is kind of my biggest benchmark 'cause of the people I interact with the most. But definitely the style of physics in Europe versus the US are very different.

1:08:28.2 CC: And I think in the US, for better or for worse, we tend to be sort of the people that always wanna make sort of wacky theories or just like, make experiments, measure things that they were not made to measure. And just kind of be, yeah, quite frankly, a bit of the dreamer is that yeah, maybe we handwave a bit and we don't worry so closely about the 15th loop correction and we're a little bit sloppy. But the fact that that then opens up the possibility of like, oh, let's get weird with it in science, I think is something that I really like about the US atmosphere in physics. So if a place like China or CERN wants to go ahead and do a machine that's like the FCCEE or the CEPC, then that's great. And the precision physics and really measuring the standard model, I think that that's very in touch with the science goals of those communities. But then doing the weird project, that's like, well, can this even work? Do we know what we're doing? This is such a far off shot, and then maybe it'll happen. To me is very in line with sort of the American ideology towards doing physics, especially looking for new physics.

1:09:41.5 CC: So that to me is kind of the ideal outcome of everything is that countries that are much better about keeping track of factors of two and pi go for the precision machine, and then the Americans pull the resources to put a muon collider on their soil. But of course, all of these projects are international.

1:09:58.3 SC: Sure.

1:10:00.4 CC: And international collaboration is not only encouraged, but required to pull any of this off. So where it ends up is not zeroth-order but it definitely, it has an impact.

1:10:12.8 SC: Yeah, I think it's a very good point. And it's not just that one style is better than the other, but a diversity of styles is very helpful.

1:10:20.4 CC: Absolutely.

1:10:21.0 SC: And especially in a situation where we don't know exactly what we will see at this next generation.

1:10:27.6 CC: Yeah. I think there's a lot of push and pull that needs to happen in the community right now is that, we need precision, but we also need discoveries. We need experimental evidence, but we also need motivating theories and having sort of the most diverse pool of ways that we can sort of approach these problems that we don't really have a clear answer to, or a clear way of proceeding on. I think that that's going to be the most robust way to find success.

1:10:52.0 SC: And I remember seeing an interview with a scientist from CERN who made a perfectly reasonable point that we're not building particle colliders that often, we built the LHC. It's gonna take a long time to build the next one. And there's a very real danger that we forget how to do it that there's a lot of implicit knowledge that just sort of ages out if we don't do this regularly.

1:11:17.6 CC: Oh, absolutely. There's no textbook that you can buy. There's no IKEA manual to how to build a particle collider. And so much of what's taught is just passed down between groups and training individuals and things like that. So I think sometimes it's a little bit... I go back and forth about how seriously I should wait this, 'cause yeah, part of it, if you just kind of sell it like that, then it feels like, okay, well you just wanna keep the field of particle physics alive so that you have a job. But it is just, it is much more profound, I think, is that if you wanna keep particle physics alive so that there's the possibility to have a collider, if we make a lot of progress in one area or another, then you need to preserve the knowledge.

1:12:03.5 SC: And good. Let's bring it, wind up sort of by bringing it back to the physics goals here. Are there things that a muon collider would be specifically good at discovering if they happen to be out there?

1:12:12.2 CC: Yeah, so I think again, this is something... I've definitely become the person that I feel like wants to have a foot, at least in both camps in terms of understanding sort of where these machines lie in terms of likelihood and progress and physics goals and things like that. So a muon collider versus an e+e- collider can have similar physics programs, but there's definitely strengths for one versus the other. So if you're just comparing e+e- and not like a future LHC at a much higher energy, the energy frontier is something completely new. So if you just wanna see new physics directly produced above a couple of 100 GEV, you need a muon collider for that. And if you wanna see it produced with fundamental particles, you can't even compare it to a proton proton machine.

1:13:03.8 CC: And just for comparison, right? If you were to build a 10 TEV muon collider, which sounds less than the LHC 'cause that's 14, but 'cause protons are composite, a 10 TEV muon collider would be comparable to the physics for the average collision that you can get out of something like a 70 or 80 TEV if not more proton proton machine. So that 100 TEV number that you might hear thrown around by China and CERN would be comparable to a 14 TEV muon collider. So the fact that these are composite particles makes super... Makes a big difference in terms of what energies are accessible. So the energy frontier, you just need it and there's a million theories that you can test. You can do things that have to do with Susie. You can do things that are just extensions of the electroweak sector. You can do things that are just completely new particles or dark matter related or things like that. And a lot of the times, you just need higher energies to really see the effects of these particles show up. So the energy frontier, I think is really the most compelling reason to say that a muon collider is something that we shouldn't invest in.

1:14:13.1 CC: In terms of other physics programs, as we said in the beginning, the Higgs boson is definitely the most mysterious particle. And depending on exactly your ideology, you may or may not say that a Higgs boson is new physics. But I'm not NEMA so I'm not gonna say that. But you can study a lot of Higgs bosons in a way that you can't do at the LHC and you can do it in a much cleaner environment in some ways too. So attend to EV muon collider, given the physics program that we hope to run, you can produce 10 million Higgs bosons. And with 10 million Higgs bosons, you can study a lot. So if you really wanna flush out the story about how the Higgs couples to itself, how it couples to other particles, how the symmetry is restored in the electroweak sector at higher energies, a muon collider is also a very good tool for that. So while the e+e- machine is usually what's billed as the Higgs factory, a muon collider could also have a lot of the complimentary and overlapping physics program of that kind of machine too. So studying the Higgs and going to higher energies to give you the soundbite, are really what we wanna do with a muon collider. But those are basically the only two things that I even know what to suggest in terms of trying to resolve things like the hierarchy problem or dark matter or stuff like that.

1:15:33.6 SC: Your day job is writing papers thinking about models of physics.

1:15:37.3 CC: That's what they tell me.

1:15:38.6 SC: Beyond the standing model. We're recording this during the day, so I guess this is part of your job, but... So, it's the end of the podcast. We can let our hair down. Do you have a specific favorite model that you have personally worked on, or thought of that might be amenable to testing in some way? Just to give the... I want the audience to know what it's like to be a working theoretical particle physicist.

1:16:04.1 CC: Okay. Well, step one, have three cups of coffee and two existential crises a day. But, okay. So my favorite model, I'll give you two answers to this. There's one model that I just hope is right and I just hope is out there just 'cause it'd be nice. So I really hope that there's a new Z prime or vector boson out there. So I want some new spin-1 particle that looks like a massive photon. We often will call that the dark photon or Z prime. I just want that thing to be out there. I think having the existence of that particle would just open so many interesting questions. And it could be the portal to dark matter. It could be the portal to something else interesting. It could be something that mixes with the electroweak sector. And there's all sorts of new complicated stuff and we have to reinterpret everything. So I really hope it's out there.

1:16:58.4 SC: So, sorry, we know of the good old Z boson, there's only one of them, it's a neutral spin-1 particle. And you're saying a different neutral spin-1 particle with a different mass, but doing kind of similar things.

1:17:11.1 CC: Yeah. Basically a heavy photon is what we're looking for. And the reason that that's exciting is 'cause whenever you have a spin-1 vector particle, it couples to something else. It's charged under something else. So it could be a portal into some whole new world of physics. And so this is the second answer that I hope that we find is, I hope that we find just an entire new sector of physics. It could be super symmetry, it could be all the super partners. It could be that dark matter is an entire sector and not just a single particle that accounts for everything. I hope there's some new group of particles and I hope they're strongly interacting 'cause I just think that that would be super fun to work on for the next 50 years So in terms of what I think is out there but I have no idea truly.

1:18:03.8 CC: I think definitely things will have to show up between the energies that we can see and the Planck scale, which is that magic number that should set the Higgs mass. Something has to be out there. I just fundamentally can't reconcile a world where there's just 16 orders of magnitude that are empty and I don't think that this is an accident. I don't think the universe is this way, 'cause some supreme power fine-tuned some numbers and look where we are. I think there has to be some deeper explanation. So whether or not the stuff shows up at sort of the TEV to 10 TEV range, there are some predictions for that. Super symmetry could be hiding in this range. There could be evidence that the Higgs boson is also a composite particle that could pop up at this energy. And really sort of the first, the first real hint that we could see of new physics that would affect the electroweak sector is really four pi times the VEV. So that sits right around a couple TEV, around three TEV. Is that a guarantee? Absolutely not, but it's a way to start. So yeah, I don't know. I just wanna see. I wanna see something that messes with the Higgs, that's what I wanna see, and whatever it is, who knows, but I think something's gotta be out there.

1:19:27.2 SC: So your calendar for the working particle physicist that includes three cups of coffee, two existential crises. It also has to include a sense of absolute grandiosity that we can figure all of this out, that it's something that is amenable to our understanding.

1:19:45.2 CC: Yeah, I think I got into particle physics at a very weird time. So I am finishing my first postdoc, which means that I've been in the field for about 10 years. And in that time, I have seen the Higgs boson discovered and supersymmetry not discovered. I have seen the Tevatron shut off and the LHC turn on, and the plans for the High Lumi, LHC to go on. And now I think people have tried to convince me throughout this entire career that I've had that particle physics is dead. We don't know what we're doing. Give up and do cosmology or biology as you suggested earlier. And I just think that can't be farther from the truth, 'cause what amazing privilege I have as a young person in this field is that I get to be part of the conversation of how we unstick ourselves, right? Is how we decide what the next generation of experiments gets to be. And we get to shape this idea of, well, how do we look for new physics? And to have my whole career, well, most of my career in front of me, as we go through this era of absolute change and really first order changes in how we're going to approach particle physics. I think it's an exciting time to be a particle physicist. I don't think that's a very popular opinion. But it is what I have to tell myself every day to get out of bed and start my existential crises and brew that first cup of coffee.

1:21:11.8 SC: With very good reasons, it sounds like. Cari Cesarotti, thanks so much for being in the Mindscape Podcast.

1:21:16.3 CC: Thanks Sean.

1 thought on “289 | Cari Cesarotti on the Next Generation of Particle Experiments”

Greetings – my comment is about your book THE BIGGEST IDEAS IN THE UNIVERSE. I’m a guy of no more than “above average” intelligence that finds topics like quantum mechanics, astronomy and molecular biology fascinating. I read a fair amount on each of these topics. its just my opinion that your book is not always easy to understand with way too much math. It reads to me more like a text book than a compelling story and narrative about our universe. It could be that your target audience are scientists in waiting – but if this is geared to the endlessly curious layman, I’d say you may have missed the mark a bit – despite your obvious mastery of the subject . Just my opinion… Jim Mc

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Scientists might achieve the impossible and actually see gravity

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September 18, 2024

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LHC experiments observe quantum entanglement at the highest energy yet

Quantum entanglement is a fascinating feature of quantum physics—the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, which has no analog in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing.

In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell, and pioneered quantum information science.

Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published in Nature , the ATLAS collaboration reports how it has succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet.

First reported by ATLAS in September 2023 and since confirmed by two observations made by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.

"While particle physics is deeply rooted in quantum mechanics , the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable," says ATLAS spokesperson Andreas Hoecker. "It paves the way for new investigations into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow."

The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study entanglement.

The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum traits to its decay particles. Physicists observe and use these decay products to infer the top quark's spin orientation.

To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018. In particular, they looked for pairs in which the two quarks are simultaneously produced with low particle momentum relative to each other. This is where the spins of the two quarks are expected to be strongly entangled.

The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted. By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations.

In its second study , currently available on the arXiv preprint server, the CMS collaboration also looked for pairs of top quarks in which the two quarks are simultaneously produced with high momentum relative to each other. In this domain, for a large fraction of top quark pairs, the relative positions and times of the two top quark decays are predicted to be such that classical exchange of information by particles traveling at no more than the speed of light is excluded, and CMS also observed spin entanglement between top quarks in this case.

"With measurements of entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible, we can test the Standard Model of particle physics in new ways and look for signs of new physics that may lie beyond it," says CMS spokesperson Patricia McBride.

Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using lepton+jets events from proton-proton collisions at √s = 13 TeV, arXiv (2024). DOI: 10.48550/arxiv.2409.11067

Journal information: arXiv , Nature

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Physical Review Accelerators and Beams

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Theory of particle beams transport over curved plasma-discharge capillaries

A. frazzitta, r. pompili, and a. r. rossi, phys. rev. accel. beams 27 , 091301 – published 13 september 2024.

No Citing Articles
INTRODUCTION
ABP SINGLE PARTICLE AND BEAM DYNAMICS
NUMERICAL SIMULATIONS
CONCLUSIONS
ACKNOWLEDGMENTS

We present a new approach that demonstrates the deflection and guiding of relativistic electron beams over curved paths by means of the magnetic field generated in a plasma-discharge capillary. The active bending plasma (ABP) represents a promising solution that has been recently demonstrated with a proof of principle experiment. An ABP device consists of a curved capillary where large discharges (of the order of kA) are propagated in a plasma channel. Unlike conventional bending magnets, in which the field is constant over the bending plane, in the ABP, the azimuthal magnetic field generated by the discharge grows with the distance from the capillary axis. This features makes the device less affected by the beam chromatic dispersion so that it can be used to efficiently guide particle beams with non-negligible energy spreads. The study we present in the following aims to provide a theoretical basis of the main ABP features by presenting an analytical description of a single-particle motion and rms beam dynamics. The retrieved relationships are verified by means of numerical simulations and provide the theoretical matrix formalism needed to completely characterize such a new transport device.

Received 7 May 2024
Accepted 20 August 2024

DOI: https://doi.org/10.1103/PhysRevAccelBeams.27.091301

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

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Physical Systems

Authors & Affiliations

1 University of Rome Sapienza , Piazzale Aldo Moro 5, 00185 Rome, Italy
2 Laboratori Nazionali di Frascati , Via Enrico Fermi 54, 00044 Frascati, Italy
3 INFN Milano , via Celoria 16, 20133 Milan, Italy

Article Text

Vol. 27, Iss. 9 — September 2024

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ABP magnetic field: field amplitude as a function of distance from the axis of the capillary: comparison between a realistic discharge field [ 10 ] (blue) and the current-equivalent Biot-Savart type field (orange). Note that the discharge field tends to a linear behavior near capillary axis.

ABP reference system: (a) capillary render and coordinates: ρ c , bending radius; r c , capillary section radius; s , curvilinear coordinate along the capillary axis; and θ , bending angle. In orange, a sketch of the ABP bent capillary with a detail on transverse section. (b) Beam particle coordinates represented on capillary transverse section: Cartesian reference system ( x , y ) with origin centered on the beam mean equilibrium radius ρ 0 , associated with the magnetic field B 0 satisfying the beam rigidity equation. In orange, capillary circular inner boundary.

Minimum required current I lim as computed from Eq. ( 3 ) as a function of beam energy, for several r c / ρ c ratio values.

ABP and CBM dispersion functions comparison, as computed from Eq. ( 8 ), for ρ 0 = 1 and k x = 10 . The dispersion amplitude difference is on the order of O ( 10 2 ) , while the period difference is O ( 10 1 ) . In the detail window below can be observed the same exact trend of the two dispersions for bending lengths s ≲ λ x / 4 : indeed, the ratio between ABP and CBM dispersion is given by a sinc function in Eq. ( 10 ) that tends to 1 for s → 0 .

ABP transition: (a) 1–6 transition space for a whole beam above transition, where regions corresponding to faster (slower) states relative to the reference trajectory are highlighted in green (blue). The presence of finite emittance broadens the transition hyperbola (orange), resulting in an excess of states at Δ s < 0 (slower beam centroid); (b) γ T limit, with possible γ T values as a function of beam energy (abscissa), where it can be observed that for γ < ρ c / r c (dashed black line), the beam will be constrained below the transition.

Beam rms elongation plotted versus beam Lorentz factor γ and transition Lorentz factor γ T from Eq. ( 18 ). Transition line γ = γ T is shown in solid red. Minimal elongation is shown in solid white. The orange region shows out-of-use configurations, where I < I lim [see Eq. ( 3 )]. The presented case features σ Δ γ / γ = 0.01 and ε n = 1 mm mrad . After some threshold energy given by emittance and energy spread, the optimal condition is found above transition. This knowledge may be relevant in ultrashort beam applications.

Transverse and longitudinal beam dynamics. Comparison between numerical and analytical solutions for 50 MeV beams. (a) Numerical rectified trajectories in bending plane, comparison between optical (upper plot, mismatched beam, no energy spread) and dispersive (lower plot, matched beam, 1% energy spread) envelope oscillations. As expected, optical oscillations happen at double frequency compared to dispersive ones. Red dashed line shows expected equilibrium radius Eq. ( 2 ). In the dispersive case, note the slight misalignment between oscillation extremes, due to the energy dependence of k x ; (b) scatter plot of deviation from the reference trajectory at the end of the evolution of an initially pointlike beam in the longitudinal coordinate and with γ > γ T , σ Δ γ / γ = 0.01 , and ε n = 10 mm mrad . The color bar shows the amplitude of each particle’s betatron oscillation, showing a clear correlation with delay respect to reference trajectory.

(a) ABP/CBM transverse spot ratio as a function of device length, evaluated for several σ Δ γ / γ . Dashed line shows expected behavior given by Eq. ( 11 ), which works properly for greater spot oscillations (e.g., 10% case). (b) Beam rms elongation as a function of energy spread, plotted for matched beams in a wide emittance range. Aspect ratio is set to unity in all cases. Dashed lines are given by Eq. ( 18 ) and show good agreement with numerics.

Transverse rms size saturation with increasing offset with respect to the equilibrium radius Δ x inj = 0 , 0.3 mm (a), (b). The beam is injected with a double rms size compared to matching, to better observe saturation in case (a). The full blue lines depict the numerical evolution of beam size, while dashed lines represent analytical predictions. The dashed red line indicates the saturation length calculated using Eq. ( 21 ), and the dashed black line represents the saturation value from Eq. ( c7 ).

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In 2022, a surprisingly high value of its mass measured by the CDF experiment plunged the particle into a "midlife crisis." The CDF W boson mass, 80,433.5 MeV with an uncertainty of 9.4 MeV ...
Scientists might achieve the impossible and actually *see* gravity
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LHC experiments observe quantum entanglement at the highest energy yet
Quantum entanglement is a fascinating feature of quantum physics—the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no ...
Phys. Rev. Accel. Beams 27, 091301 (2024)
We present a new approach that demonstrates the deflection and guiding of relativistic electron beams over curved paths by means of the magnetic field generated in a plasma-discharge capillary. The active bending plasma (ABP) represents a promising solution that has been recently demonstrated with a proof of principle experiment. An ABP device consists of a curved capillary where large ...