PEN experiment: a measurement of $\pi^+\to e^+\nu_e(\gamma)$ branching ratio

The experimental $\pi^+\to e^+\nu_e(\gamma)$ decay branching ratio currently provides the most accurate test of lepton universality. The PEN experiment at PSI, Switzerland, aims to improve the present world average experimental precision of $\Delta B/B = 3.3\cdot 10^{-3}$ to $\sim$$5\cdot 10^{-4}$ using a stopped pion beam. During runs in 2008-2010, PEN has acquired over $2\cdot 10^7$ $\pi_{e2}$ events. The experiment includes active beam detectors (degrader, mini TPC, target), central MWPC tracking with a plastic scintillator hodoscope, and a spherical pure CsI electromagnetic shower calorimeter. We will present a progress report on the PEN analysis. In addition to $\pi_{e2}$ and the normalizing $\pi\to\mu\to e$ process, we will discuss radiative pion and muon decays, decays in flight, as well as accidental and hadronic backgrounds.