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.

The weak interaction was proposed by Fermi in 1933, to interpret the beta decay. The interaction Lagrangian is the product of two charged currents (CC) – one of the nucleons, one of the leptons. It was later discovered that parity and charge conjugation are not conserved and that the structure of the charged currents is a combination of vector and axial currents, V–A. The beautiful Goldhaber experiment on the helicity of the neutrino.

The coupling of all leptons is universal, but not that of the quarks. To obtain universality, Cabibbo introduced the concept of mixing of the hadronic currents, namely of quarks. Then the Glashow–Iliopoulos–Maiani mechanism solved a problem introducing the hypothesis that a fourth quark would exist, the charm, completing a doublet with the strange one. With the discovery of two more quarks, the quark mixing matrix contains a phase factor that is the origin of CP violation in the Standard Model.

The weak neutral currents were discovered with the Gargamelle bubble chamber at CERN in 1973. This showed a close similarity between weak and electromagnetic interactions and opened the way to their unification.

This chapter presents the last interaction described by the Standard Model of particle physics, i.e. the weak interaction. A historical approach is followed, trying to explain the evolution of its theoretical description from the experimental discoveries: we start from Fermi theory before introducing the charged and neutral currents. The mixing matrices both in the quark sector and in the leptonic sector are described. The phenomenon of neutrino oscillation is also detailed. The chapter concludes with a detailed discussion of CP violation.