Fifty years of particle physics research has produced an elegant and concise theory of particle interactions at the subnuclear level. This book presents the experimental foundations of that theory. A collection of reprints alone would, perhaps, have been adequate were the audience simply practicing particle physicists, but we wished to make this material accessible to advanced undergraduates, graduate students, and physicists with other fields of specialization. The text that accompanies each selection of reprints is designed to introduce the fundamental concepts pertinent to the articles and to provide the necessary background information. A good undergraduate training in physics is adequate for understanding the material, except perhaps some of the more theoretical material presented in smaller print and some portions of Chapters 6, 7, 8, and 12, which can be skipped by the less advanced reader.

Each of the chapters treats a particular aspect of particle physics, with the topics given basically in historical order. The first chapter summarizes the development of atomic and nuclear physics during the first third of the twentieth century and concludes with the discoveries of the neutron and the positron. The two succeeding chapters present weakly decaying non-strange and strange particles, and the next two the antibaryons and the resonances. Chapters 6 and 7 deal with weak interactions, parity and CP violation.

While the existence of antiparticles was established with Anderson's discovery of the positron in 1932, it was not clear in 1955 whether the pattern of each fermion having an antiparticle, suggested by the Dirac equation, would hold for baryons, the heavy particles p, n, Λ, Σ, and. There were two arguments raising doubts about such particles. One was that nucleons had an anomalous magnetic moment that differed markedly from the Dirac moment. Measurements by Otto Stern in 1933, later improved by I. I. Rabi, had shown that the proton had a magnetic moment of 2.79 nuclear magnetons. [One nuclear magneton is eh/(2Mpc), where Mp is the nucleon mass.] The neutron's magnetic moment, which would be zero if the neutron were an ordinary Dirac particle, was measured by L. Alvarez and F. Bloch in 1940 to have a value of −1.91 nuclear magnetons. The second reason was based on a cosmological argument. Where were the antigalaxies one expected if the Universe had baryon–antibaryon symmetry?

One of the motivations for the choice of the energy for the Bevatron was the hope that the antiproton could be found. The momentum chosen, 6.5 GeV/c, was above threshold for antiproton production on free protons, p + p → p + p + p +, to occur.

The most enigmatic of elementary particles, neutrinos were postulated in 1930, but were not observed until a quarter of a century later. It took another forty years to determine that they are not massless.

Neutrinos are a ubiquitous if imperceptible part of our environment. Neutrinos created in the Big Bang together with the cosmic background radiation pervade the entire Universe. The Sun is a poweful source of MeV neutrinos. Neutrinos in the GeV range are created when cosmic rays strike the atmosphere, 15 kilometers or so above the Earth's surface. Every nuclear reactor emits antineutrinos copiously. High-energy neutrinos are regularly produced at accelerators through particle decay and carefully fashioned magnetic fields can focus produced unstable charged particles to create neutrino beams.

Traditionally, efforts were made to set upper limits on the masses of the neutrinos associated with the electron, muon, and tau lepton. As explained in Chapter 6, if the electron neutrino were sufficiently massive the electron spectrum in tritium beta decay would be distorted near the end point. This prompted many painstaking measurements over the past thirty years. The expression for the spectrum actually depends on the square of the neutrino mass and the best fits can return unphysical, negative values for this.