The PIONEER Rare Pion Decay Experiment*

PIONEER is a recently approved, next-generation, rare-pion decay experimental program at the Paul Scherrer Institute in Switzerland. The first phase of the experiment will focus on a measurement of the charged-pion branching ratio to electrons vs. muons R_e/μ as a test of lepton flavor universality to be performed at an order of magnitude greater sensitivity than any other experiment. It is strongly motivated by a variety of anomalies in flavor physics, including the Muon g-2 results, several B-quark decays and even the current tension with the unitarity of the quark mixing matrix CKM. At present, the SM prediction for R_e/μ is known to 1 part in 10⁴, which is 15 times more precise than the current experimental result. An experiment reaching the theoretical accuracy will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles up to the PeV mass scale. Later measurements of the rare process of pion beta decay, π⁺ → π⁰e^₊ν, with 3 to 10-fold  improvement in precision, will determine the CKM matrix element V_ud in a theoretically pristine manner and test CKM unitarity. Various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity. The experiment is based on a novel conceptual design. A high intensity beam will be delivered by an upgraded beamline, pions and muons will be tracked in a segmented low gain avalanche detector (LGAD) stopping target, and positrons detected by trackers and  a 25 radiation length liquid xenon (or LYSO) calorimeter. Compared to the  previous generation of rare pion decay experiments, the 4-D tracking capability of the active target allows for excellent separation of the π⁺ → e⁺ν signal from the dominant background from π⁺→ μ⁺ν followed by normal muon decay.