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 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.

This chapter introduces the first fermion generation. We begin with the electron and the left-handed neutrino, their CP invariance as well as anomalies in triangle diagrams and Witten’s global SU(2) anomaly. They are both canceled by adding up and down quarks. We discuss the constraints that anomaly cancelation imposes on the electric charges of the fermions. Finally we also add a right-handed neutrino, extend the anomaly discussion to the lepton and baryon numbers, and further extend the model by proceeding to technicolor.