Determination of Low Energy $\pi\to e\nu$ Electromagnetic Tail via Multivariate Cut Analysis

The PEN experiment at the Paul Scherrer Institute aims to measure the leptonic decay $\pi^+\to e^+\nu(\gamma)$ branching ratio with $5\cdot 10^{-4}$ relative uncertainty. Electromagnetic shower leakage for 70 MeV monoenergetic positrons in the pure Csi electromagnetic calorimeter is of the order of 2\,\% for $E_{CALO}<50\,$MeV and presents an important systematic uncertainty. We have explored a set of cuts via multivariate analysis to determine the best algorithm for extracting the almost background-free set of $\pi_{e2}$ tail events.