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

A new measurement of Be 2, the + ! e + ( ) decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on Be 2 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 e2 decay, the branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.