Speaker
Description
PIONEER is a next-generation rare pion decay experiment approved to take place at the Paul Scherrer Institute (PSI). Its primary goal is to measure the branching ratio of charged pion decays into electrons versus muons to an accuracy of 0.01%. Achieving this goal would improve on the current experimental precision by more than an order of magnitude and match the precision of the Standard Model prediction, providing a powerful test of lepton flavor universality. Additional physics goals include testing CKM matrix unitarity and searching for weakly coupled new physics. The heart of the experiment is a highly segmented active target (ATAR) built from low-gain avalanche diodes (LGADs), which stops the incident pions and provides 5D $(x, y, z, t, E)$ information on the pions and their decay products. Surrounding the ATAR is a deep, large-acceptance, segmented electromagnetic calorimeter composed of LYSO crystals, which measures the decay positron energy to distinguish $\pi \to e\nu$ from $\pi \to \mu \to e$ events. This talk will present the status of the PIONEER experiment, highlighting this year's test beam campaigns at PSI to characterize the pion beam and test an ATAR demonstrator.