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Introduction Even a superficial comparison of the twelfth Solvay conference on physics, held in Brussels in October 1961 to discuss the quantum theory of fields, with the eighth Solvay conference, held in September 1948 to discuss elementary particles, highlights the dramatic changes in particle physics that had taken place in the decade of the 1950s. The rapporteurs at the later conference were Richard Feynmen, Murray Gell-Mann, Marvin L. Goldberger, Walter Heitler, Gunnar Källen, Stanley Mandelstam, Abraham Pais, and Hideki Yukawa, all of whom (except for Heitler and Yukawa) received their graduate training in the 1940s. The list of invited members included a large number of younger theoreticians: Geoffrey Chew, Marcello Cini, Freeman Dyson, Tsung Dao Lee, Francis Low, Harry Lehmann, Yoichiro Nambu, Abdus Salam, Julian Schwinger, Leon Van Hove, Arthur Wightman, and Chen Ning Yang. This was in marked contrast with the 1948 conference (organized in the fall of 1947, and scheduled to be held in April 1948). There, some thirty-nine physicists were invited – the youngest being Homi J. Bhabha, who was thirty-eight at the time. There were few Americans on the invited list, and all these were émigrés (besides Albert Einstein, only Hans Bethe, Felix Bloch, Enrico Fermi, Bruno Rossi, and Edward Teller), because the resources of the Solvay Institute could not cover travel expenses from the United States. Everyone invited was a member of the prewar élite – among the theoreticians were Niels Bohr, Paul A. M. Dirac, Hans A. Kramers, Oscar Klein, Wolfgang Pauli, Rudolph Peierls, and Erwin Schrödinger. [ABSTRACT FROM AUTHOR]

I should like to begin by expressing regret for not having been able to be present for the last Fermilab meeting, at which I had been asked to give the final talk about what it was like to be a student of theoretical physics in the late forties, the end of the period covered by that meeting. I still hope to present that material somewhere. Between that conference and this one there were two others, one in Paris in the summer of 1982, where I gave a talk about my experiences with strangeness, and one in 1983, in Sant Feliu de Guíxols in Catalunya, where I spoke on the subject “Particle Theory from S-Matrix to Quarks.” That second conference was called a “trobada” in the Catalan language, a word that reminds us of the troubadours of the Middle Ages who flourished in Catalunya. It reminds us also that we have become much like those medieval minstrels. We spend a great deal of time now traveling from one orgy of reminiscence to another, each one held in the capital of some princely state. Here I am helping to close another “trobada.” Let me repeat something I said in Catalunya: I shall once again commit the sin of hindsight in the eyes of some of those historians of science who come from the historical tradition. [ABSTRACT FROM AUTHOR]

Introduction The first Rochester conference on high-energy nuclear physics was held 16 December 1950, twenty months after the last of the three Shelter Island conferences on the foundations of quantum mechanics, organized by Robert Oppenheimer (1947–9). Five high-energy accelerators (“high energy” was defined as > 100 MeV thirty-five years ago!) had been completed since the end of World War II and were producing research results: two at Berkeley (340–MeV synchrocyclotron and 300-MeV electron-synchrotron), the Rochester 240-MeV and Harvard 150-MeV synchrocyclotrons, and the Cornell 300-MeV electron-synchrotron. Many more accelerators with even higher energies were under construction. Cosmic-ray experimentalists throughout the world were making extremely important contributions to the field. It seemed clear to me that the Shelter Island conferences – which had been limited to a small number of theorists, with a couple of “token” experimentalists – should be replaced by a new series of conferences that would give equal weight to the participation of accelerator experimentalists, cosmicray experimentalists, and theorists. As the newly installed chairman of the Rochester Physics Department – with one of the few meson-producing machines in operation and a very active cosmic-ray group – I decided to organize an invitational conference that would thoroughly discuss the latest developments in high-energy physics. The first two Rochester conferences were small (fifty to seventy-five participants), were of short duration (two days), were supported by local industry, and still were not truly international (Europe and Japan were recovering from the war). Their success and the increasing worldwide interest in the field led to the raising of sights in all respects. [ABSTRACT FROM AUTHOR]

It was in the spring of 1952 that I joined the Tata Institute of Fundamental Research (TIFR) as a student. The shining light in theoretical research was Homi Bhabha, who had then completed his work on cosmic-ray cascades and relativistic wave equations. Bhabha was deeply involved in organizing a nuclear research team in Bombay, but he was still very much the academic leader and presided over the weekly colloquia. He also brought to Bombay many distinguished physicists, like Paul A. M. Dirac, Maria Goeppert-Mayer, Maurice Levy, Robert Marshak, Joseph E. Mayer, Abraham Pais, Wolfgang Pauli, and Sin-itiro Tomonaga, and he convinced Bernard Peters to join the TIFR faculty. I worked with Peters, and I quickly graduated from developing and scanning nuclear emulsions to developing theoretical aspects of experimental particle physics. Several τ mesons were discovered in our laboratory; for myself, I began to study in detail Richard Dalitz's paper on τ decay analysis. Enrico Fermi's thermodynamic model and Werner Heisenberg's shockwave model of multiple meson production were of great interest at TIFR. A large “star” was discovered in an emulsion stack; the study of the distribution and interaction of secondaries was among my first lessons in particle physics. But all this time the glamour topics were relativistic wave equations and quantum theory of higher-spin fields. During my second year in Bombay, Marshak came to lecture at TIFR and told us about the Chicago experiments on pion–nucleon scattering and the 3-3 resonance. This was the beginning of an association that has had a decisive effect on my scientific career. [ABSTRACT FROM AUTHOR]

Never in my fondest dreams did I expect in January 1957 to be invited, twenty-eight years later, to reminisce about decisive experiments on symmetry violation in a great national laboratory directed by Leon M. Lederman. Such reminiscences not only are unnecessary (some of the key protagonists having already published their recollections in Maglich's nowdefunct journal) but also can be misleading. Nobody has total recall, and personal souvenirs can get warped by things heard but not experienced. Just recall Einstein's contradictory answers to the question whether he was or was not aware of the Michelson-Morley experiment in 1905. Theoretical prologue My friend Laurie Brown has asked me to include in my talk some remarks about the history of the theoretical developments in the field of weak interactions, as several of the main contributors to this field are present here but have chosen not to talk. I shall oblige Laurie Brown in some sense, but still avoid giving a chronological presentation devoid of personal reminiscences. Such presentations are available in standard textbooks. Let me not praise famous men: It might be embarrassing (or insufficient) to them and to me. Let me instead mention a few early contributions that, for one reason or another, have rarely been cited in past reviews. Before the famous Lee–Yang paper on parity nonconservation, theorists generally stated that there existed five different four-fermion couplings. [ABSTRACT FROM AUTHOR]

It is a great pleasure to contribute to this volume on the history of particle physics in the 1950s as one of the experimental physicists who participated in the establishment of the puzzle, one that eventually was so constrained by experiment that it had to have a far-from-trivial resolution. The period I shall discuss starts in the summer and fall of 1953, when the properties of what we now call K mesons were being established, and concludes in the summer of 1956 with the publication of the Lee–Yang paper on parity violation. What always paces the rate of discovery is the development of new instruments – new devices for probing phenomena, either with a new level of precision or by extending the range of observation into realms never before explored. The growth of our knowledge of K mesons is a superb example of this. The new knowledge was acquired at the rate at which new devices were invented, developed, and applied to the problems at hand. Figure 31.1 gives my impression of some of the important instrument developments and the time when they became useful laboratory tools. Chronological lists like this one have appeared elsewhere. Perhaps the only thing new on the present list is the development of the end-window phototube. While in itself it seems like a minor thing, it had a tremendous impact in the utility of both scintillation and Cerenkov counters. All of us who tried to design a Cerenkov counter using the old 931-A or 1P21 phototubes will understand what I mean. [ABSTRACT FROM AUTHOR]

The period of discovery The first K-meson decay process to become established was the τ+ decay mode, observed in 1948 by Cecil F. Powell's group at Bristol in emulsion exposed to the cosmic radiation at mountain altitudes. That was only a year after they had discovered and identified the π+ meson, Yukawa's nuclear-force meson, through its decay mode π+ → μ+ + v. The τ+ decay mode was soon confirmed by J. Brian Harding's report in 1950 of a second event of this type, with a compatible release of kinetic energy. In the next few years, further events were found, both by these and by other groups, until there were eleven events of mean total mass 495 ± 2 MeV available for comparison at the Royal Society Discussion Meeting held at London, 29 January 1953. The second K-meson decay process to become established was the x+ decay mode, reported by Cormac O'Ceallaigh in 1951, also from observations in emulsion. Because the charged secondary came to rest and decayed, it was identified uniquely; its measured kinetic energy was only 5.9 MeV. No other identified events of this type were reported until 1954, when three further events were described, their μ+ kinetic energies being 33.0 MeV, 28.0 MeV, and 13.6 MeV. This spread of μ+ energies implied that there were at least two neutral particles emitted in x+ decay. [ABSTRACT FROM AUTHOR]

Introduction In 1928 and 1930, two experiments, performed by Richard Cox, C. G. Mcllwraith, and Bernard Kurrelmeyer, and by Carl Chase, provided, at least in retrospect, evidence for the nonconservation of parity in the weak interactions. The anomalous nature of the experimental results seems to have been fairly well known, although the exact nature of the anomaly was not clear. One thing is certain, however; the relation of these results to the conservation of parity was not recognized or understood by any contemporary physicists, including the authors themselves. In this chapter, we examine these experiments in some detail and discuss whether or not they really provided evidence for the nonconservation of parity. Did the experiments show the nonconservation of parity? The intellectual context for these experiments was the desire to demonstrate the vector nature of electron waves. Louis de Broglie had suggested in 1923 that just as light exhibited both particle and wave characteristics, so should what we normally consider particles exhibit wave characteristics. This wave nature had been brilliantly confirmed in 1927 in the experiment on the diffraction of electrons by crystals performed by Clinton J. Davisson and Lester H. Germer. The idea of electron waves was then combined with the concept of electron spin, introduced by George Uhlenbeck and Samuel Goudsmit. The mathematics of a vector electron were worked out by Charles G. Darwin. Cox and his collaborators thought that an experiment in which electrons were double-scattered would provide evidence, in analogy to experiments with light and x rays, that the first scattering polarized the light and the second scattering acted as an analyzer. [ABSTRACT FROM AUTHOR]

Background and introduction About ten years ago, a group of sociologists and scientists gathered at Cornell University in a new program of research designed to discover and validate new methods for studying the evolution and health of scientific fields using quantitative measures. The interests of the group's founders lay both in finding ways to obtain a better basic understanding of how scientific fields develop – how, for example, the social and intellectual aspects of a scientific community interact – and in producing work that might have an influence on the way in which science policy is formulated by showing policy analysts better ways of monitoring the outputs of science and relating them to the inputs. We believed that the best way to proceed with this program was to undertake several case studies of scientific specialties using teams of collaborators made up of sociologists and scientists familiar. with the specialties in question. We believed, further, that one case study should focus on a field with a strong and highly developed theoretical basis that also involved major experimental work. That meant a field like particle physics, and we thought that within particle physics the study of the weak interactions might be a manageable specialty on which to focus. A large and comprehensive data base was constructed, comprised of all serial articles published in the specialty from 1950 to 1975 (N = 5,765), and our own weak-interactions citation index was constructed from the refer ences that appear at the end of the serial articles. Over 80,000 references, originating in one of the articles in our weak-interactions bibliography (citing articles both within our bibliography and in other fields), make up our citation index. [ABSTRACT FROM AUTHOR]

The “Gruppenpest” that so many physicists fought against before the war died out during the 1940s. Although some famous physicists made pioneering, fundamental contributions in the theory of infinite-dimensional representations of groups, the community of physicists ignored them.* However, in the late forties, with the discovery of new particles (π mesons and V particles), the need arose to know more about conservation laws. For instance, that a spin-1 boson cannot decay into two photons was known from a short, brilliant paper by Lev Landau and from Eugene P. Wigner's un-published findings, quoted by Jack Steinberger. Using field-theory concepts, particle physicists were eager to establish the list of selection rules for particle decays; for example, C. N. Yang and D. C. Peaslee gave their proof of spin 1 ↛ 2γ. In a long review I wrote in 1951, I carefully referenced all that was known. A complete list of selection rules for angular momentum and parity conservation appeared only later. Then came the θ – τ puzzle. Its history is recounted excellently by Richard Dalitz in Chapter 30. Apparently this was a typical case of the parity doublet; T. D. Lee and Yang described it in an elegant paper, six months before they proposed parity violation as the explanation. So much has been written on the history of P violation (and some contributions to this conference are devoted to it) that I wish here only to mention some “prehistoric” relevant publications. Pierre Curie, in a fundamental and classic paper (in which he formulated his famous symmetry principles), explained parity conservation and the difference between axial and polar vectors in classical physics. [ABSTRACT FROM AUTHOR]

The efforts that came to fruition with the detection of the free neutrino started at Los Alamos in 1950–1, when several of us who were engaged in the testing of nuclear bombs wondered if that man-made star could be used to advance our knowledge of physics. For one thing, the unusual object featured lots of fissions, followed by β decays; so why not use it as a source of neutrinos? After all, it is an extraordinarily intense pulse, and signals produced by such a source might be sufficiently copious and distinguishable from background. Some hand-waving and rough estimates led me to conclude that the bomb was, in fact, the best source. All that was required was a massive detector, measuring a cubic meter or so. As it happened, Enrico Fermi was at Los Alamos during the summer of 1951, and I took the opportunity to talk with him about these matters. The conversation went something as follows: “I would like to talk with you a few minutes about the possibility of neutrino detection.” He was very pleasant, and said, “Well, tell me what is on your mind.” “First off as to the source,” I said, “I think that the bomb is best.” After a few moments thought he replied, “Yes, the bomb is the best source.” So far so good. And then I said, “But one needs a detector which is so big and I don't know how to make such a detector.” He thought about it further and said he didn't know either. [ABSTRACT FROM AUTHOR]

The neutrino, neutron, positron, and muon were discovered in the 1930s. The pion and strange particles had their turn in the late forties. During the fifties, which saw the change from dependence on natural sources to accelerators in the study of particles, the particles and their properties were sorted out, the strange particles were systematized, the first particle resonances were seen, and parity was found to be violated, with a concomitant clarification of the weak interaction. It was an exciting time. The field advanced rapidly. I was young and played my part in this adventure. Cosmic rays and muon decay For me, particle physics began in 1947, when I was a graduate student at the University of Chicago. Enrico Fermi gave a seminar on the results of the Conversi, Pancini, and Piccioni experiment in which he explained that because the negative mesotrons rapidly find their way to the atomic K shell, the observed long lifetimes were incompatible with the role of these mesons as Yukawa particles. It was a beautiful and important experiment, and Fermi's explanation was extraordinarily lucid, as well as stimulating and exciting. The puzzle was at least in part cleared up a few months later when the Bristol group discovered the pion in nuclear emulsions. It was a great privilege to be a student in that department at that time. There were excellent teachers: Fermi, Edward Teller, Willie Zachariasen, and Maria Mayer, to name a few. Fermi's courses, in particular, were models of transparent and simple organization of the most important concepts. [ABSTRACT FROM AUTHOR]

My plans at Brookhaven Cosmic-ray physicists had been dreaming of observing antiprotons long before the construction of the Bevatron. In fact, some events that could be interpreted as revealing antiprotons had already been observed in cloud chambers and in photographic emulsions, but the indications were not compelling. Being a cosmic-ray physicist, I shared that “antimatter rush” when I was at Brookhaven working with the Cosmotron. At the Brookhaven cafeteria, discussing antiprotons was the order of the day. Hartland Snyder and Maurice Goldhaber bet $400 on whether or not the antiproton existed. Goldhaber lost. In contrast to this, physicists from Berkeley had hardly any interest in antiprotons. They considered the wish to discover the antiprotons unoriginal, if not naive. After all, the energy of the Bevatron was the bare minimum for the production of antiprotons, and no one could be confident about it. Moreover, the hot subjects at the time were the strange particles (copiously produced by the Bevatron), which had opened a field of physics monopolizing the attention of the theorists, with the K0, K1 and K2 quartet, and the experiments that enabled T. D. Lee and C. N. Yang to perceive the parity violation. Even as late as December 1954, there was so little interest in the antiproton that at the American Physical Society (APS) meeting in Berkeley no one presented a paper speculating on its discovery. However, I thought that even a slight chance of finding antimatter justified a great effort. [ABSTRACT FROM AUTHOR]

The Bevatron started operating in early 1954 at what was then the Radiation Laboratory and is now known as the Lawrence Berkeley Laboratory. Some personal background Sula (Sulamith Goldhaber) and I came to Berkeley from Columbia University in 1953. I joined the Physics Department and Emilio Segrè's group at the Radiation Laboratory. She joined Walter Barkas's group and later Edward Lofgren's group. We had been working with photographic emulsions at Columbia's cyclotron located at Nevis, with the help and encouragement of Gilberto Bernardini. Before that, I used emulsions loaded with D2O as a γ-ray spectrometer for my Ph.D. thesis under Hugh Richards at the University of Wisconsin in Madison. Setting up with photographic emulsions Whereas in my earlier work I had used 100–600-μm single small emulsions on glass, this was the period in which emulsion stacks began to be used in cosmic-ray work at Bristol and elsewhere; the electron-sensitive emulsions had recently been introduced by Kodak Ltd. of England, followed by C. Waller at Ilford, in close consultation with Cecil F. Powell and Giuseppe P. S. Occhialini. Thus, I started out at Berkeley to build up an emulsion-processing plant in the Physics Department – the photographic-emulsion arm of the Segrè group. That involved new techniques for marking emulsion sheets, to allow easy track following from sheet to sheet, the modification of microscopes with special stages to hold and manipulate those large emulsion sheets after they were mounted precisely on glass, and the construction of accurate microscope stages for multiple scattering measurements. [ABSTRACT FROM AUTHOR]

Between 1947 and 1963, a period freely referred to here as “the fifties,” elementary particle physics was growing up and undergoing the triumphs and pains of its adolescence. Beginning with the discovery of charged pions in cosmic rays, the period ended with the proposition that the basic constituents of the hadrons are quarks. The period witnessed the vindication of quantum field theory in renormalized quantum electrodynamics (QED), only to see it rejected as a theory of the strong and weak interactions. It saw the concept of symmetry emerge as a fundamental characteristic of basic physics, followed by its downfall in the parity revolution. Researchers discovered particles that were so different from any known previously that they labeled them “strange,” and they found unexpected properties in familiar interactions. The fifties was a time of intellectual turmoil, of shifting attitudes and changing theoretical fashions. For particle physics, as for science in general, the period was a time of transition, as energy was redirected from the problems of war to the concerns of peace. Profiting from their participation in the radar and atomic bomb projects (we are speaking now mainly of the United States, but that is where the shift began), physicists planned and carried out cooperative scientific research on a scale previously unknown in peacetime and drew on the prestige that derived from their wartime successes to obtain the necessary funding. Elementary particle physics, then called, significantly, “high-energy nuclear physics,” served as a prototype for other large-scale scientific projects, such as the space programs in the United States and the Soviet Union. [ABSTRACT FROM AUTHOR]