New studies of allowed pion and muon decays

TL;DR

The PEN experiment aims to precisely measure the pion decay branching ratio B_{πe2} to test lepton universality and search for non-Standard Model physics, building on previous PIBETA results.

Contribution

This work reports a new high-precision measurement of B_{πe2} using the PEN detector, significantly reducing experimental uncertainty and enhancing tests of lepton universality.

Findings

Over 2×10^7 π_{e2} events collected

Expected improved precision of B_{πe2} measurement

Enhanced constraints on non-(V-A) physics contributions

Abstract

Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_{\pi e2} = R^\pi_{e/\mu}, the \pi^+ -> e^+\nu(\gamma) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40\times experimental precision lag behind theory to ~ 6-7\times. Because of large helicity suppression, R^\pi_{e/\mu} is uniquely sensitive to contributions from non-(V-A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B_{\pi e2} provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2\times 10^7 \pi_{e2} events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative \pi and \mu decays.