Flavour universality of the $W^\pm$ and $Z$ fermionic couplings

Abstract

The Standard Model does not provide any dynamical explanation of the existence of different families of fermions. To account for this experimental fact, it just replicates three times its single-family gauge structure. The equal-charge fermions of the different families couple to the gauge bosons with exactly the same coupling strength. We overview the empirical evidence supporting this important property. The currently most precise experimental tests on the universality of the lepton and quark couplings are discussed. Both charged-current ($W^\pm$) and neutral-current ($Z$) interactions are reviewed.