Chapter 6, “A Deeper Layer of Reality,” describes my path to quarks, relating events starting as a graduate student in the spring of 1963 through the summer of 1964 when my work on quarks was essentially completed. A way of judging improbable theories is presented that, when applied to the quark model, pits the a priori likelihood that quarks exist against the difficulty of explaining the experimental data in a theory without quarks.

After a preliminary discussion of symmetry as applied to particle classification, and the constraints it places on the wave functions of particles, a detailed discussion of constituent quarks with spin is presented, based on my 1964 Erice Summer School Lectures. This takes place at two levels, first to capture the main ideas, then, with more detail, to enable the reader to decide if they would have believed that a fundamental theory based on quarks would eventually explain the strong interactions. Selection rules governing the change of strangeness in weak decays, and their relation to the change in charge of the strongly interacting particles, are derived. A graphical calculus based on quarks for calculating hadron couplings is introduced.

Chapter 5 focuses on Murray Gell-Mann who dominated particle physics for more than a decade starting in the mid-1950s. His perspective, style, and major contributions to physics, while I knew him, are described. A comparison of Feynman and Gell-Mann’s views on how to practice physics, and what they valued concludes this chapter.

A succession of toy field theories of increasing generality are described, the final one, missing all strong interactions, is based on mathematical quarks from which equal-time commutation relations of the weak and electromagnetic currents are abstracted. The Eightfold way and the Gell-Mann—Okubo mass formula are discussed, and Gell-Mann’s view of quarks is described in some detail. Examples of a darker side -- his pattern of inadequate attribution, that I only fully realized while writing this book -- are also given.

Chapter 4 on Richard Feynman, my theoretical physics thesis advisor, is a collection of vignettes that reveal aspects of behavior and thought that contributed to his mystique and unique accomplishments in physics.

After relating the history behind Feynman’s V-A theory of party violation, much of it in Feynman’s own words, the rest of the chapter is based on my personal interactions with Feynman lasting for a little more than twenty years, from the time I arrived at Caltech in 1959 till I left in 1981. Feynman’s attitude towards experimental results related to parity violation provides an informative background to how he would handle experimental information related to the discovery of quarks. The intent here, and in the remainder of the Chapter, is to give the reader a sense of how Feynman thought about physics, how he practiced it, and what he valued. His struggle with constituent quarks (aces), and what to make of them, lasted considerably longer than a decade, passing though several phases, including one with partons, but eventually ending with his fully accepting their reality.

Chapter 7, “Epilogue,” looks back on the discovery of quarks, identifies what of the original conception has survived, and what was missing. It also revives ideas that led to the discovery of quarks, long forgotten, that are relevant today. Lessons learned are highlighted.

The concepts underlying quantum chromodynamics (QCD), the quantum field theory based on quarks and gluons, are summarized. The discovery of the fourth ace (quark) in 1971 in Japan, and Petermann’s 1965 paper (“Properties of Strangeness and a Mass Formula for Vector Mesons”) are described. The split A2, exotics (tetraquarks and pentaquarks), the quark-gluon plasma, and the possibility of free fractionally charge particles are briefly recounted.

Chapter 2 chronicles the explosion in the number of strongly interacting particles, and efforts to understand them. It ends with an introduction to the discovery of quarks (originally called “aces”), and the resistance to accepting them for what they are: real particles that live in a deeper layer of reality.

The concepts of quantum number, resonance, and scattering cross section are explained, and the theories meant to explain the existence of strongly interacting particles are elucidated, including Fermi and Yang’s composite pion, Sakata’s composite hadrons, Chew and Frautschi’s “bootstrap,” and Heisenberg’s nonlinear spinor theory. The discovery of quarks suggested by the anomalous suppression of phi decay is detailed, and the importance of anomalies in physics is highlighted. Two remarkable meson and baryon mass relations are given. Both positive and negative reactions to the idea of quarks as constituents of hadrons are presented. Chapters 1 and 2 describe the recurring chaos and confusion that existed during the time between the discoveries of radioactivity and quarks. Once discovered, the path to the acceptance of quarks as real particles was equally confusing.

Chapter 3 on Alvin Tollestrup, my experimental physics thesis advisor, describes the singular contributions he made to physics, and what was required to practice experimental particle physics at the highest level. What I learned from him affected me profoundly, giving me the understanding of experiments necessary for the discovery of quarks.

Tollestrup: Developed photomultipliers as particle detectors to obtain the most acute values of the masses of the light elements. Designed the RF system and a million-volt pulse transformer to inject electrons into Caltech’s Synchrotron that observed the first pion-nucleon resonance beyond its peak. Found the pion’s beta decay into an electron and neutrino at CERN, removing the last obstacle to the acceptance of the V-A theory of parity violation. Performed the first users group experiment at the Bevatron. Designed the first superconducting magnets for Fermilab’s Tevatron. Helped convert the Tevatron to a proton-antiproton collider, the most powerful collider for 25 years until the LHC at CERN was constructed. For this work he received the National Medal of Technology  and Innovation.