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.