# PEN: a low energy test of lepton universality

## TL;DR

The paper reviews the current experimental status of pion electronic decays, highlighting their importance for testing lepton universality and the Standard Model, and discusses the PEN experiment's potential for improved measurements.

## Contribution

It provides a comprehensive review of pion decay experiments and discusses the PEN experiment's prospects for enhancing experimental precision in testing lepton universality.

## Key findings

- Current experimental precision is much lower than theoretical predictions.
- Limits on non-Standard Model processes are constrained by existing measurements.
- The PEN experiment aims to significantly improve the accuracy of pion decay measurements.

## Abstract

Allowed charged $\pi$ meson decays are characterized by simple dynamics, few available decay channels, mainly into leptons, and extremely well controlled radiative and loop corrections. In that sense, pion decays represent a veritable triumph of the standard model (SM) of elementary particles and interactions. This relative theoretical simplicity makes charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for studying pion structure and chiral dynamics. Even after considerable recent improvements, experimental precision is lagging far behind that of the theoretical description for pion decays. We review the current state of experimental study of the pion electronic decay $\pi^+ \to e^+\nu_e(\gamma)$, or $\pi_{e2(\gamma)}$, where the $(\gamma)$ indicates inclusion and explicit treatment of radiative decay events. We briefly review the limits on non-SM processes arising from the present level of experimental precision in $\pi_{e2(\gamma)}$ decays. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.