PEN: a sensitive search for non-(V-A) weak processes

TL;DR

This paper discusses a new measurement of the pion decay branching ratio, which tests lepton universality and is sensitive to potential new physics beyond the Standard Model.

Contribution

It reports on a novel experimental effort to measure the $B_{ ext{pi e2}}$ decay with higher accuracy, aiming to improve tests of lepton universality.

Findings

Current measurement is the most accurate test of lepton universality for this decay.

The decay's helicity suppression makes it sensitive to new physics contributions.

The experimental accuracy still lags behind theoretical predictions by over an order of magnitude.

Abstract

A new measurement of $B_{\pi e2}$, the $\pi^+ \to e^+\nu(\gamma)$ decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on $B_{\pi e2}$ constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Thanks to the large helicity suppression of $\pi_{e2}$ decay, the branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.