James Cronin The Nobel Prize in Physics 1980

autobiography and work

work

Since the beginning of particle physics, discrite symmetries seemed to play a crucial role and to be true for all the particle interactions. The parity (P) symmetry stated that a “mirror” universe would have the same physical laws as ours, the charge conjugation parity ( C ) would state that a universe in which all the particles passed to antiparticles would still have the same physical laws and the time inversal (T) symmetry stated that a universe in which time flew in the other direction would still have the same physical laws. It seemed a strong reason. After all, why would nature prefere one side over the other, may it be direction up or down, matter over antimatter or time forward or backward?

Only that in 1957, Madame Wu made a breaktrough discovery, proving there is P violation in a beta decay of a Co 60 nucleus. The discovery was accepted right away, as there was no other alternative explanation. Nature preferes one direction over the other in this particular decay, nature can distinguish over the left and the right in the direction given by the spin of the Co atom. Very quickly, C violation was also proved and the field of particle physics was in shock. A theory was then proposed, that could integrate the C violation, the P violation, but stated that CP together was conserved.

That means, the physical laws in a mirror universe in which all the particles flipped to antiparticles would still be the same as in our universe. This model is now called the V-A theory and was developed in 1957-58. Of course, the model had to be set to experimental evidence. The ? decays showed large C violation, large P violation, but CP held perfectly. There was a huge succes and everybodey was happy.

Moreover, the fondation of quantum field theory (QFT) was a theorem stating that all three symmetries toghether will hold no matter what.

The fact that T would therefore hold too was a reassuring one.

It is in this cotext that in 1963 two stuborn researchers (Val Fitch - Princeton University and James Cronin – Chicago University) decided to check again the validity of this new CP conservation theorem. Almost 5 years had passed since the new V-A theory had been tested and accepted by the physical community. They were not taken seriously by their collegues, who found the subject uninteresting.

They decided to study the K meson system. Previously, neutral kaon decays had been analyzed. 300 decays, no sign of CP violation, so an upper limit of a possible CP violation was set. They wanted to study 7500 events of Kaon decays. They expected to ameliorate this upper limit from 1/300 to 1/7500. But, to their luck, nature had made other plans. Every 1000 events, 2 or 3 CP violating events would happen. Indeed, they could see almost 14 events and indeed they saw events.

They published the results the next year, in 1964. Again, the physical community was shocked. But this time, not like in the P violation discovery, alternative theories were

proposed to explain the phenomenon and save the CP conservation theory. No less than 10 such theories were proposed and…tested and they all failed. CP violation was accepted in a few years as a reality of nature.

It would only happen in the weak interaction, it would have two forms, indirect – the one discovered by Fitch and Cronin- and direct, seen only 3 decades afterwards. Nowadays, CP violation has been seen only in the K meson system and a few years ago in the B meson system, at the B-factories.

It is very important to understand this CP violation, as it sets the complex phase in the CKM matrix, which sets parameters on the Standard Model, the unitary triange. It could explain why matter dominates over antimatter in our universe, whereas in the Big Bang they were created in equal quantities, it could play a role in the dark matter (that would be a weakly interacting matter) and shed light on cosmology too. All pieces would fall toghether at the end.

This is why their discovery was very important and was awarded a Nobel Prize, which they shared, in 1980. And now, after the brief overview of the whole picture, let’s see the theory behind this!

Theory of CP violation

The K meson comes as a 0 K and its antiparticle 0 K . But they are not eigenvectors of the CP operator, as this one transforms the particle into its own antiparticle. 0 0 ) ( K K CP + = 0 0 ) ( K K CP + = But their sum and their difference would be eigenvectors of this operator. ) ( ) ( 0 0 0 0 K K K K CP + + = + ) ( ) ( 0 0 0 0 K K K K CP ? ? = ? As CP is conserved in general, the only physical particles would be those that are eigenvectors of the CP operator. This is why the real effective physical quantities are not the 0 K and its antiparticle 0 K , but K 1 and K 2 . ) ( 2 1 0 0 2 K K K ? = ) ( 2 1 0 0 1 K K K + = These kaons decay. If they respect CP conservation, than the K 1 that has CP= +1 would decay into 2 ?, whe reas the K 2 with CP= -1 would decay into 3?, as the pion has CP= -1. These decays have indeed been seen and they were a confirmation of the CP Page 3 conservation.Richard Feynman wrote at the time that explaining the decay in the kaon system was one of the greatest achievements in physics! K 1 proved to be a short lived particle (2-3 cm) and was called K S (K short) and K 2 proved to live a lot more (about 15 m) and was called K L (K long).

Now Fitch and Cronin came into the game. In a few words, their experiment was the following. They take a beam of protons, collide it onto a target, create a lot of stuff, mostly hadronic, take one direction to create a beam, use a magnet to sweep away all the charged particles, keep a neutral beam (that is mostly made up of neutrons with impurity of neutral kaons, thus of K S and K L. We pass this beam through the purest vacuum we can make and in a meter’s distance we are sure that all the K S are gone. We only have the K L if we reconstruct 3? and obtain the mass of the K L, we know we have a K L -> 3?, if we reconstruct 2? and obtain the mass of the K L , then we have seen an event of K L ->2?, an event forbidden if CP is conserved. This is because the mass mass difference is very very small. That’s how they proved CP violation does exist! Let’s see more in detail how they discovered the CP violation! The experiment of Fitch and Cronin (1963-1964). Un upper limit of an eventual CP violation had been established before their experiment to 1/300 for the fraction of K L that would decay into 2? instead of 3?. Fitch and Cronin wanted to ameliorate this upper limit by analysing more events of K L decay. Their experiment was done at Brookhaven AGS .

It was a fixed target experiment, in which the beam was made of protons of 30 GeV energy and the target was an internat one of Be. Berillium is a very light atom (Z=4), this is why it is very attractive for fixed target experiments. It would assure that the produced particles are the lightest ones in which we are interested and not rests of more massives nuclei or heavier particles. Particles are created in all directions, but they chose only the one going at 30 degrees with respect to the initial proton direction. They used a collimator of 1.5:1.5:48 in at a distance of 14.5 ft.

from the internal target. They wanted a neutral beam, so they needed a sweeping magnet to deviate all the charged particles. The magnetic field was 512 kG-in and was located at 20 ft. from the internal target. Another collimator is present at 55 ft. froom the internal target to focalize the beam before the decay volume. What was then their neutral beam made from?

There were no gamma rays, because the photons were absorbed by a plaque of Pb of 1.5 in. in thickness placed in front of the first collimator. The neutrons were not stopped how ever, as only the thermalized neutrons (the very slow ones) interact with the nucleus, the fast one simply get through. Our beam also contains 0 K and 0 K , which are linear combinatins of K L or K S. As the K S lives for about 3 cm, after 1 m they were sure there was no K S left. Their beam was a beam of neutrons impurified with the K L , but it is these K L that they were interested in. One needs to avoid regeneration of K S that happens when K L passes through matter and one needs to limit the interactions of Page 4 neutrons with matter. One needs to take a very low Z material as possible or even create a vacuum. They chose a He gas at STP conditions ( atm p C t 1 , 0 = ° = ).

The detector itself is made up of two arms of spectrometer, each one made up of 2 spark chambers, with a magnetic field of 178 kG-in.between them. They are followed by a Cerenkov counter and a scintillation chamber as a trigger. When a signal comes simultanously from these two last detectors, the spark chamber is triggered. When they use the K S regeneration for the calibration of the detector, an anticoincidence counter was placed right away after the regenerator.


autobiography
I was born on September 29, 1931 in Chicago, Illinois, while my father, James Farley Cronin, was a graduate student at the University of Chicago. He was a student of classical languages. My mother, Dorothy Watson, had met my father in a Greek class at Northwestern University. After a brief stay at a small school in Alabama, my father became Professor of Latin and Greek at Southern Methodist University in Dallas, Texas, in September 1939. My primary and secondary education was provided by the Highland Park Public School System. I received my undergraduate degree from Southern Methodist University with a major in physics and mathematics in 1951. In high school my natural interest in science was encouraged by an excellent physics teacher, Mr. Charles H. Marshall. He stressed analytical methods as applied to simple physical systems as well as practical experimental problems. My real education began when I entered the University of Chicago in September 1951 as a graduate student. I was fortunate to have among my classroom teachers, Enrico Fermi, Maria Mayer, Edward Teller, Gregor Wentzel, Val Telegdi, Marvin Goldberger and Murray Gell-Mann. I did a thesis in experimental nuclear physics under the direction of Samuel K. Allison. While at Chicago my interest in the new field of particle physics was stimulated by a course given by Gell- Mann, who was developing his ideas about Strangeness at the time.

It was also at the University of Chicago that I met my future wife, Annette Martin, in the summer of 1953. It was a wonderful, happy summer; I had passed my Ph.D. qualifying exams the previous winter, and I realized that I had met my lifetime companion. We were married in September 1954. The stable point in my life became our home. On even the worst days, when nothing was working at the lab, I knew that at home I would find warmth, peace, companionship, and encouragement. As a consequence, the next day would surely be better. Annette, with great patience and good spirit, tolerated my many long absences when experiments were carried out at distant laboratories.

After receiving my Ph.D. in 1955 I had the opportunity to join the group of Rodney Cool and Oreste Piccioni who were working at the Brookhaven Cosmotron, a newly completed 3 GeV accelerator. That period was an exciting time in physics. The famous tau-teta puzzle led to the prediction of parity violation and the experimental demonstration of its violation. The long-lived K meson was discovered at Brookhaven. When the violation of parity was discovered I began a series of electronic experiments to investigate parity violation in hyperon decays. In early 1958 the Cosmotron suffered a severe magnet failure. As a consequence, we moved our experiment to the Berkeley Bevatron. Here I had the good fortune to meet William Wenzel and Bruce Cork. These physicists had a great influence on me. From their example I learned not to be intimidated by complex pieces of apparatus. While at Brookhaven I met Val Fitch who was responsible for my coming to Princeton University in the fall of 1958. At Princeton all the work in particle physics was supported through a contract with the Office of Naval Research. The Director of the Laboratory, George Reynolds, was most supportive of my efforts to work independently. There followed for ten years a glorious time for research. I was much involved in the development of the spark chamber as a practical research tool. During this period, with a series of excellent students, we further studied hyperon decays. Then we joined with Val Fitch to study neutral K meson decays which led to the discovery of CP violation.

Following the discovery in the summer of 1964, I spent a year in France working at the Centre d'Etudes Nucleaires at Saclay with Rene Turlay. In addition to the research, I enjoyed learning French and assimilating the culture of another country. One of the greatest joys in my life was giving a lecture in French at the College de France. On returning to Princeton in 1965, I began with students a series of experiments to study the neutral CP violating decay modes of the long lived neutral K meson. These experiments lasted until 1971. In 1971 I returned to the University of Chicago as Professor of Physics. The fact that the new Fermilab 400 GeV Accelerator was being built near Chicago made this move an attractive one. At Fermilab, with younger associates and students, I carried out experiments on the production of particles at high transverse momentum, and on the production of direct leptons. At present with my colleague at Chicago, Bruce Winstein, I am preparing to study with much greater accuracy some of the CP violating parameters of the neutral K meson.

I now live in Chicago near the campus with my wife Annette, and son Daniel. My oldest daughter Cathryn lives and works in New York City. My daughter Emily attends the University of Minnesota. My mother remained in Dallas, Texas, after the death of my father in 1959. For recreation we have a cabin in the woods in Wisconsin which we visit year-round. In the summer we spend some time in Aspen, Colorado. Our whole family assembles in Chicago at Christmas and usually in Aspen in the summer. Education B.S., Southern Methodist University, 1951 M.S., University of Chicago, 1953 Ph.D., (Physics) University of Chicago, 1955

Career National Science Foundation Fellow, 1952-1955 Assistant Physicist, Brookhaven National Laboratory, 1955-1958 Assistant Professor of Physics, Princeton University, 1958-1962 Associate Professor of Physics, Princeton University, 1962-1964 Professor of Physics, Princeton University, 1964-1971 University Professor of Physics, University of Chicago, 1971