PEN Experiment: Towards a New Measurement of the Pienu Branching Ratio

The experimental determination of the $\pi^ + \to e^+\nu(\gamma)$ decay branching ratio currently provides the most accurate test of lepton universality. The PEN experiment at PSI, Switzerland, aims to improve the experimental precision of $3.3 \times 10^{-3}$ to $5 \times 10^{-4}$ using a stopped beam approach. During runs in 2008-10, PEN has acquired over $2 \times 10^{7}$ $\pi_{e2}$ events. The experiment includes active beam detectors (degrader, mini TPC, target), central MWPC tracking with plastic scintillator hodoscopes, and a spherical pure CsI electromagnetic shower calorimeter. A progress update on key aspects of the analysis will be presented.