PIONEER is a next-generation rare pion decay experiment proposed at the Paul Scherrer Institute (PSI). In the first phase of the experiment, a measurement of the charged-pion branching ratio to electrons vs. muons (Re/μ) will be extremely sensitive to new physics effects. At present, the prediction of the Standard Model of Particle Physics for Re/μ is known to 1 part in 10^4, which is 15 times more precise than the current experimental result. An experiment reaching the theoretical accuracy will test lepton flavor universality at an unprecedented level, probing mass scales up to the PeV range. In a second phase, a measurement of pion beta decay with 3 to 10-fold improvement in sensitivity will determine the Vud term of the CKM matrix in a theoretically pristine manner and could bring decisive insight on the Cabbibo angle anomaly. The experiment design benefits from experience with the recent PIENU and PEN experiments at TRIUMF and PSI. Excellent energy and time resolutions, greatly increased calorimeter depth, high-speed detector and electronics response, large solid angle coverage, and complete event reconstruction are all critical aspects of the approach. In this talk, I will discuss the physics case of the experiment, review its conceptual design and present the ongoing R&D effort toward building the detector for the first phase.
