Optimization of background suppression in the identification of pion decays.

The pion electronic decay π_e2γ branching ratio R^π_e/μ = Γ(π→eν(γ)/π→μν(γ) ) provides the best test of electron-muon universality, a standing hypothesis of the Standard Model (SM).

Violation of this hypothesis would indicate "new" physics beyond the Standard Model. The PEN collaboration has the goal to measure R^π_e/μ to a relative precision of 5 × 10^-4 in order to bring the experimental precision closer to that of the theoretical SM calculations. The experiment consists of active beam and target detectors, thin cylindrical hodoscope array, particle tracking detectors for incoming and outgoing charged particles, as well as a pure CsI electromagnetic shower calorimeter for decay particle energy detection. This talk will focus on the techniques used to separate the two leptonic pion decay channels, and to identify background events, such as decays in flight, hadronic interactions, and Bhabha scattering. Reliable

identification of all above processes is essential for the R^π_e/μ analysis. We will discuss the impact on the biggest systematic challenge in the analysis, accurate characterization of the

low energy tail in the response of the electromagnetic calorimeter.