Speaker
Description
A precise determination of the pion electronic decay branching ratio $\Gamma(\pi\rightarrow e\bar{\nu}(\gamma))/\Gamma(\pi\rightarrow \mu \bar{\nu}(\gamma))$ provides the best test of electron-muon unversality, taken as valid in the Standard Model. Currently, the experimentally determined value of this ratio is lags behind the theoretical predicted value by an order of magnitude in precision. The PEN collaboration has performed detailed measurement of pion decays at the Paul Scherrer Institute with the goal of obtaining a relative uncertainty of $5\times 10^{-4}$ for the branching ratio of the $\pi^+\rightarrow \text{e}^\nu(\gamma)$ decay. The PEN apparatus detects pion decays at rest in the active target. The detector consists of active plastic scintillating beam counters, target, and hodoscope as well as charged particle tracking detectors and large solid angle pure CsI electromagnetic calorimeter. Among the key systematics are timing, particle tracking efficiencies, decays in flight, the acceptances, and the low-energy tail resulting from electromagnetic shower energy leakage out of the CsI calorimeter. A comprehensive review of the analysis to date including the relevant competing decay channels and systematics will be discussed.
