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

TL;DR

The paper surveys experimental tests of flavour universality for the electroweak couplings of $W^{\pm}$ and $Z$ bosons, across leptons and quarks. It compiles precise measurements from leptonic decays, semileptonic transitions, and neutral-current processes, and interprets them through ratios of couplings $g_\ell$, $v_f$, and $a_f$ within the SM and potential new-physics frameworks. The main result is that leptonic charged-current couplings are universal to about $10^{-3}$, axial $Z$ couplings are universal at the per-mil to percent level, and quark couplings are universal at the few-percent level, with CKM unitarity tests broadly supporting universality though a small first-row tension exists. The work highlights how different observables probe distinct aspects of the underlying flavour structure and outlines how EFT analyses can expose potential new dynamics should any deviation emerge, guiding future high-precision tests.

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.

Figures (2)

• Figure 1: Ratios of transition amplitudes sensitive to the ratios of leptonic couplings $g_\ell/g_{\ell'}$.
• Figure 2: Combined LEP and SLC measurements of the effective vector and axial-vector $Z$ couplings ALEPH:2005ab.