Precision tests of lepton flavor universality — a window to BSM physics

Lepton flavor universality (LFU) is an accidental symmetry of the standard model (SM), conserved in all its gauge interactions.  Small LFU violation is expected due to non-universal interactions with the Higgs field arising from non-zero and non-degenerate masses of charged leptons.  Left-handed neutrino mixing, embodied in the so-called Pontecorvo–Maki–Nakagawa–Sakata matrix ($U_{\text{PMNS}}$), leads to LFU violation in charged current leptonic interactions.  An enhancement beyond these low LFU violation levels would indicate new physics, beyond the standard model (BSM).

Experimentally, LFU violation can be tested most precisely in leptonic and semileptonic decays of light pseudoscalar mesons.  We will explore the current status of the field and the prospects for new results in the near future, with a particular focus on the $\pi^+ \to e^+\nu_e(\gamma)$ channel.