Rare pion decays at PIONEER

"The physics motivation and the conceptual design of the PIONEER experiment, a next-generation rare pion decay experiment testing lepton flavor universality and CKM unitarity, are described. Phase I of the PIONEER experiment, which was proposed and approved at Paul Scherrer Institut, aims at measuring the charged-pion branching ratio to electrons vs. muons, Re/μ, 15 times more precisely than the current experimental result, reaching the precision of the Standard Model (SM) prediction at 1 part in 10^4. Considering several inconsistencies between the SM predictions and data pointing towards the potential violation of lepton flavor universality, the PIONEER experiment will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles up to the PeV mass scale.  In addition, various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity. The later phases of the PIONEER experiment aim at improving the experimental precision of the branching ratio of pion beta decay (BRPB), pi+ -> pi0 e+ ν(γ), currently at 1.036(6)×10^-8, by a factor of three (Phase II) and an order of magnitude (Phase III). Such precise measurements of BRPB will allow for tests of CKM unitarity in light of the Cabibbo Angle Anomaly and the theoretically cleanest extraction of |V_ud| at the 0.02% level, comparable to the deduction from superallowed beta decays." [1][2]

[1] (Collaboration Proposal), “Testing Lepton Flavor Universality and CKM Unitarity with Rare Pion Decays in the PIONEER experiment”, FERMILAB, FERMILAB-CONF-22-328-PPD, 2021 Snowmass Summer Study, Seattle, WA, United States, March 2022, https://cds.cern.ch/record/2806788 [cds.cern.ch]

[2] (Collaboration Proposal), “PIONEER: Studies of Rare Pion Decays”, CERN, March 2022, https://cds.cern.ch/record/2824583 [cds.cern.ch]

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