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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]

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]

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]

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]