For physicists who study elementary particles and quantum field theory, the 1970s was a golden age. It saw the experimental confirmation of the electroweak theory, and the extension of that thinking would lead us to a successful theory of strong interactions as well. All the fundamental forces of nature, except for gravity, would be unified in what became known as the “Standard Model.” By the end of 1973, there was some experimental verification of the electroweak theory. Weinberg agrees to write The First Three Minutes, which was published in 1977. Louise visits Stanford Law School, accompanied by Weinberg, who finds his host department cold. In 1977, he collaborates with Ben Lee of Fermilab, who tragically died in a car accident later that year. Louise is invited to teach at University of Texas Law School, in the summer of 1979, after which she was offered a full professorship. The Weinbergs taught in their respective universities and met in Cambridge in the holidays. Weinberg’s Nobel Prize, shared with Salam and Glashow, is announced in October 1979, ahead of the ceremony that December.

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.