Jack Steinberger The Nobel Prize in Physics

autobiography

WORKS

Neutrinos were first detected in 1956 by Fred Reines of the University of California at Irvine and the late George Cowan. They showed that a nucleus undergoing beta decay emits a neutrino with the electron, a discovery that was recognized with the 1995 Nobel Prize for Physics. Ever since the discovery, physicists have wanted to know whether neutrinos have mass.

Direct attempts to determine the mass of the neutrino have been based on the conservation of energy and momentum in, for example, the beta decay of tritium. But results have only provided upper limits on the mass. The new evidence instead depends on a fascinating quantum mechanical process known as neutrino oscillations. To explain this we first need to know that neutrinos are elementary particles from the family of leptons. All leptons have a spin of 1/2, but some (like the electron) are charged, while neutrinos have no charge. Three types of neutrinos exist, and are distinguished by the way in which they interact. The neutrino emitted in beta decay is now called the electron neutrino, and the others are the muon and tau neutrinos.

The muon neutrino is emitted along with the muon - a charged lepton like the electron but 200 times heavier - in the decay of the pion. The muon neutrino was first detected by Leon Lederman, Jack Steinberger and Mel Schwartz at the Brookhaven National Laboratory in 1962, a discovery for which they shared the Nobel prize in 1988. The tau neutrino has never been detected directly but is known to be emitted in the decay of the tau, the heaviest lepton. A neutrino oscillation means that one type of neutrino gradually transforms into another as it moves. Indications of such oscillations have been reported before, but the new work provides the first convincing proof of their existence.

The collaboration of 120 US and Japanese scientists measured neutrinos produced in the atmosphere by cosmic rays. SuperKamiokande, a Cerenkov detector containing 50 000 tonnes of ultrapure water and located a kilometre below ground in the Kamioka mine near Takayama, can detect electron and muon neutrinos but not tau neutrinos. Neutrinos can enter the detector from either above or below - neutrinos from above travel a relatively short distance through the atmosphere, whereas neutrinos from below travel much further through the Earth. Since the cosmic-ray flux is known to be the same from both directions (except for a small geomagnetic effect), and since neutrinos interact so weakly that they penetrate the Earth, it was expected that the same number of neutrinos would enter the detector from both directions. But the SuperKamiokande team found that the number of muon neutrinos entering the detector from below was half the number coming from above. The electron neutrinos, however, were unaffected.

The only explanation for this finding is that muon neutrinos had oscillated into tau neutrinos, which cannot be detected by SuperKamiokande. The distance required for one oscillation must be between 100 and 10 000 km, which means that neutrinos travelling through only the atmosphere would not experience significant oscillations, while neutrinos also travelling through the Earth have a large probability for oscillation.

If neutrinos do have mass, it might be expected that each type of neutrino should have a different mass and a different mass eigenstate. However, nearly all theories of neutrino mass predict that the three types of neutrinos are well defined mixtures of several mass eigenstates. As time passes or the neutrino moves, the relative phases of the different components in this mixture change. Consequently, a state that is originally, say, muon-type, gradually transforms into another type of neutrino. Neutrino oscillations therefore result from the mass of the neutrino. This quantum mechanical argument is similar to that for an electron orientated at an angle to a magnetic field. In this case the electron is in a mixture of spin-up and spin-down eigenstates, and it precesses around the magnetic field. For the neutrinos, the length of oscillation (analogous to the precession time) depends on the mass difference between the eigenstates. The SuperKamiokande result suggests that both the muon and tau neutrinos are made up of approximately equal mixtures of two mass eigenstates. The mass difference between these eigenstates is such that the squares of the masses differ by 10-2-10-3 (eV)2.

The discovery of neutrino mass is significant for several reasons. For example, the pioneering work of Raymond Davis of the University of Pennsylvania since the 1960s has prompted many experiments to measure neutrinos produced in nuclear reactions in the Sun. These experiments have consistently yielded a neutrino flux less than half that calculated from solar models. It has been thought for some time that the only simple explanation for this anomaly was in terms of neutrino oscillations that convert electron-type neutrinos to another type. To relate this to the atmospheric result, there must be a third mass eigenstate, very close to one of the other two, that is a mixture of the electron neutrino with the muon and tau neutrinos. The mass difference needed to explain the solar neutrinos is much smaller than that needed to account for the atmospheric result. These three mass eigenstates can explain the "disappearance" of both solar and atmospheric neutrinos, but they are not directly related.

The SuperKamiokande result could also be important for big bang theory, which predicts that the universe contains a large background of neutrinos. If the neutrino mass was 1 eV, this would suggest that neutrinos account for more mass in the universe than all of the protons and neutrons put together. Neutrinos would therefore represent a significant amount of "dark matter", mass that we cannot see but is predicted to exist by cosmological models. The simplest interpretation of the solar and atmospheric results is that the heaviest neutrino has a mass of 0.1 eV. However, neutrino oscillations depend only on the differences in mass, so it is possible that all three masses are 1 eV or greater but that the mass differences are much smaller. Thus it is not clear how this new evidence relates to the dark matter problem. But the biggest prize may be the impact the result could have on the development of grand unified theories, which attempt to explain the weak, electromagnetic and strong interactions in terms of a single interaction. A key component of these theories is the symmetry between quarks and leptons. Indeed, there are fundamental similarities between these particles. For a start, both quarks and leptons have a spin of 1/2 and experience the same weak interactions. Furthermore, both types of particle have three families or generations. The first generation of quarks comprises the up and down quarks, the constituents of protons and neutrons. The second generation consists of the strange and charm quarks and is 10-100 times heavier, while the third and heaviest generation comprises the bottom quark and the recently discovered top quark. Similarly the three families of leptons are the electron, muon and tau, and their associated neutrinos.

In certain grand unified theories this underlying symmetry becomes exact at some high-energy scale. These models often assume that neutrinos acquire mass in the same way as other particles do. A very interesting possibility - postulated in 1977 by Murray Gell-Mann, Pierre Ramond and Dick Slansky - is that the neutrino masses are inversely proportional to the high-energy scale at which the quark-lepton symmetry is broken. If this so-called "see-saw" mechanism is true, this measurement of the neutrino mass may be the first window into new physics at energies far beyond those accessible with particle accelerators.

I was born in Bad Kissingen (Franconia) in 1921. At that time my father, Ludwig, was 45 years old. He was one of twelve children of a rural 'Viehhändler' (small-time cattle dealer). Since the age of eighteen he had been cantor and religious teacher for the little Jewish community, a job he still held when he emigrated in 1938. He had been a bachelor until he returned from four years of service in the German Army in the first World War. My mother was born in Nuremberg to a hop merchant, and was fifteen years the younger. Unusual for her time, she had the benefit of a college education and supplemented the meagre income with English and French lessons, mostly to the tourists which provided the economy of the spa. The childhood I shared with my two brothers was simple; Germany was living through the post-war depression. Things took a dramatic turn when I was entering my teens. I remember Nazi election propaganda posters showing a hateful Jewish face with crooked nose, and the inscription "Die Juden sind unser Ungluck", as well as torchlight parades of SA storm troops singing "Wenn's Juden Blut vom Messer fliesst, dann geht's noch mal so gut". In 1933, the Nazis came to power and the more systematic persecution of the Jews followed quickly. Laws were enacted which excluded Jewish children from higher education in public schools. When, in 1934, the American Jewish charities offered to find homes for 300 German refugee children, my father applied for my older brother and myself. We were on the SS Washington, bound for New York, Christmas 1934.

I owe the deepest gratitude to Barnett Faroll, the owner of a grain brokerage house on the Chicago Board of Trade, who took me into his house, parented my high-school education, and made it possible also for my parents and younger brother to come in 1938 and so to escape the holocaust. New Trier Township High School on the well-to-do Chicago North Shore, enjoyed a national reputation, and, with a swimming pool, athletic fields, cafeteria, as well as excellent teachers, offered horizons unimaginable to the young emigrant from a small German town. The reunited family settled down in Chicago. We were helped to acquire a small delicatessen store which was the basis of a very marginal income, but we were used to a simple life, so this was no problem. I was able to continue my education for two years at the Armour Institute of Technology (now the Illinois Institute of Technology) where I studied chemical engineering. I was a good student, but these were the hard times of the depression, my scholarship came to an end, and it was necessary to work to supplement the family income.

The experience of trying to find a job as a twenty-year-old boy without connections was the most depressing I was ever to face. I tried to find any job in a chemical laboratory: I would present myself, fill out forms, and have the door closed hopelessly behind me. Finally through a benefactor of my older brother, I was accepted to wash chemical apparatus in a pharmaceutical laboratory, G.D. Searl and Co., at eighteen dollars a week. In the evenings I studied chemistry at the University of Chicago, the weekends I helped in the family store.

The next year, with the help of a scholarship from the University of Chicago, I could again attend day classes, so that in 1942 I could finish an undergraduate degree in chemistry. On 7 December 1941, Japan attacked the United States at Pearl Harbor. I joined the Army and was sent to the MIT radiation laboratory after a few months of introduction to electromagnetic wave theory in a special course, given for Army personnel at the University of Chicago. My only previous contact with physics had been the sophomore introductory course at Armour. The radiation laboratory was engaged in the development of radar bomb sights; I was assigned to the antenna group. Among the outstanding physicists in the laboratory were Ed Purcell and Julian Schwinger. The two years there offered me the opportunity to take some basic courses in physics. After Germany surrendered in 1945, I spent some months on active duty in the Army, but was released after the Japanese surrender, to continue my studies at the University of Chicago. It was a wonderful atmosphere, both between professors and students and also among the students. The professors to whom I owe the greatest gratitude are Enrico Fermi, W. Zachariasen, Edward Teller and Gregor Wentzel. The courses of Fermi were gems of simplicity and clarity and he made a great effort to help us become good physicists also outside the regular class-room work, by arranging evening discussions on a widespread series of topics, where he also showed us how to solve problems. Fellow students included Yang, Lee, Goldberger, Rosenbluth, Garwin, Chamberlain, Wolfenstein and Chew. There was a marvellous collaboration, and I feel I learned as much from these fellow students as from the professors.

I would have preferred to do a theoretical thesis, but nothing within reach of my capabilities seemed to offer itself. Fermi then asked me to look into a problem raised in an experiment by Rossi and Sands on stopping cosmic-ray muons. They did not find the expected number of decays. After correcting for geometrical losses there was still a missing factor of two, and I suggested to Sands that this might be due to the fact that the decay electron had less energy than expected in the two-body decay, and that one might test this experimentally. When this idea was not followed, Fermi suggested that I do the experiment, instead of waiting for a theoretical topic to surface. The cosmic-ray experiment required less than a year from its conception to its conclusion, in the end of the summer of 1948. It showed that the muon's is a three-body decay, probably into an electron and two neutrinos, and helped lay the experimental foundation for the concept of a universal weak interaction. There followed an interlude to try theory again at the Institute for Advanced Study in Princeton, where Oppenheimer had become director. It was a frustrating year: I was no match for Dyson and other young theoreticians assembled there. Towards the end I managed to find a piece of work I could do, on the decay of mesons via intermediate nucleons. I still remember how happy Oppenheimer was to see me come up with something, at last.

In 1949, Gian Carlo Wick, with whom I had done some work on the scattering of polarized neutrons in magnetized iron while still a graduate student at Chicago University, invited me to be his assistant at the University of California in Berkeley. There the experimental possibilities in the Radiation Laboratory, created by E.O. Lawrence, were so great that I reverted easily to my wild state, that is experimentation. During the year there, I had the magnificent opportunity of working on the just completed electron synchrotron of Ed McMillan. It enabled me to do the first experiments on the photoproduction of pions (with A.S. Bishop) to establish the existence of neutral pions (with W.K.H. Panofsky and J. Stellar) as well as to measure the pion mean life (with O. Chamberlain, R.F. Mozley and C. Weigand).

I survived only a year in Berkeley, partly because I declined to sign the anticommunist loyalty oath, and moved on to Columbia University in the summer of 1950. At its Nevis Laboratory, Columbia had just completed a 380 MeV cyclotron; this, for the first time, offered the possibility of experimenting with beams of T mesons. In the next years I exploited these beams to determine the spins and parities of charged and neutral pions, to measure the pi- pi0 mass difference and to study the scattering of charged pions. This work leaned heavily on the collaboration of Profs. D. Bodansky and A.M. Sachs, as well as of several Ph.D. students: R. Durbin, H. Loar, P. Lindenfeld, W. Chinowsky and S. Lokanathan. These experiments all utilized small scintillator counters. In the early fifties, the bubble-chamber technique was discovered by Don Glaser, and in 1954 three graduate students, J. Leitner, N.P. Samios and M. Schwartz, and myself began to study this technique which had not as yet been exploited to do physics. Our first effort was a 10 cm diameter propane chamber. We made one substantial contribution to the technique, that was the realization of a fast recompression (within ~10 ms), so that the bubbles were recompressed before they could grow large and move to the top. This permitted chamber operation at a useful cycling rate. The first bubble-chamber paper to be published was from our experiment at the newly built Brookhaven Cosmotron, using a 15 cm propane chamber without magnetic field. It yielded a number of results on the properties of the new unstable (strange) particles at a previously unattainable level, and so dramatically demonstrated the power of the new technique which was to dominate particle physics for the next dozen years. Only a few months later we published our findings on three events of the type Sigma0-> Delta0 + gamma, which demonstrated the existence of the Sigma0 hyperon and gave a measure of its mass. This experiment used a new propane chamber, eight times larger in volume, and with a magnetic field. This chamber also introduced the use of more than two stereo cameras, a development which is crucial for the rapid, computerized analysis of events, and has been incorporated into all subsequent bubble chambers.