PEN experiment: a precise measurement of the pi+ -> e+ nu decay branching fraction

TL;DR

This paper reports on a precise measurement of the pion decay branching ratio to test lepton universality and explore potential new physics effects, with current results being the most accurate experimental test to date.

Contribution

It presents the most accurate experimental measurement of the $ ext{π}^+ o e^+ u( ext{γ})$ decay branching ratio, crucial for testing lepton universality and probing new physics.

Findings

Current measurement is the most precise test of lepton universality.

The decay's branching ratio is sensitive to potential new physics contributions.

Results highlight the need for further precision to match theoretical predictions.

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. Because of the large helicity suppression of the $\pi_{e2}$ decay, its branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.