A new measurement of $B_{pi e2}$, the $pi^+ to e^+ u(gamma)$ 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 availa
ble. 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.
A new measurement of $B_{pi e2}$, the $pi^+ to e^+ u(gamma)$ 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 availa
ble. 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.
The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu branching ratio at the Paul Scherrer Institute, with the goal uncertainty of delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all published pie2 decay measur
ements lags behind the theoretical calculation by a factor of 40. In this contribution we report on the PEN detector configuration and its performance during two development runs done in 2007 and 2008.
