Testing Lepton Flavor Universality with Pion, Kaon, Tau, and Beta Decays

TL;DR

This paper reviews current experimental and theoretical efforts to test lepton flavor universality through low energy decays, exploring implications for physics beyond the Standard Model and future research directions.

Contribution

It provides a comprehensive overview of experimental results, theoretical status, and global fits related to lepton flavor universality violations in low energy decays.

Findings

Current experiments show potential deviations from universality.

Global fits constrain new physics contributions.

Future experiments could clarify possible anomalies.

Abstract

We present an overview of searches fo violation of lepton flavor universality with focus on low energy precision probes using pions, kaons, tau leptons, and nuclear beta decays. The current experimental results are reviewed, the theoretical status within the context of the Standard Model is summarized, and future prospects (both experimental and theoretical) are discussed. We review the implications of these measurements for physics beyond the Standard Model by performing a global model-independent fit to modified $W$ couplings to leptons and four-fermion operators. We also discuss new physics in the context of simplified models and review Standard Model extensions with focus on those which can explain a possible deviation from unitarity of the Cabibbo-Kobayashi-Maskawa quark mixing matrix.