Documents

Publications and preprints of the collaboration

• Testing Lepton Flavor Universality and CKM Unitarity with Rare Pion Decays in the PIONEER experiment,
  arxiv:2203.05505, contribution to Snowmass 2021, (March 2022)
  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, R_e/μ, 15 times more precisely than the current experimental result, reaching the precision of the Standard Model (SM) prediction at 1 part in 10⁴. 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. The later phases of the PIONEER experiment aim at improving the experimental precision of the branching ratio of pion beta decay (BRPB), π⁺→π⁰e⁺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.
• PIONEER: Studies of Rare Pion Decays, 
  arXiv:2203.01981, Proposal, (March 2022)
  A next-generation rare pion decay experiment, PIONEER, is strongly motivated by several inconsistencies between Standard Model (SM) predictions and data pointing towards the potential violation of lepton flavor universality. It will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles even if their masses are at very high scales. Measurement of the charged-pion branching ratio to electrons vs. muons R _e/μ is extremely sensitive to new physics effects. 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 test lepton flavor universality at an unprecedented level, probing mass scales up to the PeV range. Measurement of pion beta decay, π⁺→π⁰e⁺ν(γ), with 3 to 10-fold improvement in sensitivity, will determine |V _ud| in a theoretically pristine manner and test CKM unitarity, which is very important in light of the recently emerged tensions. In addition, various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity. The experiment design benefits from experience with the recent PIENU and PEN experiments at TRIUMF and the Paul Scherrer Institut (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. The PIONEER experiment design includes a 3π sr 25 radiation length calorimeter, a segmented low gain avalanche detector stopping target, a positron tracker, and other detectors. Using intense pion beams, and state-of-the-art instrumentation and computational resources, the experiments can be performed at the PSI ring cyclotron.
   
• Proposal submitted to the Research Committee (January 2022)
   
• Presentation during BVR 53 at PSI (January 2022)