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 (SM) 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 SM 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 Vud 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.
Bio-sketch:
Quentin Buat is an acting assistant professor at the University of Washington, working on the ATLAS experiment at CERN and the PIONEER experiment. He completed his PhD in 2013 at the Université de Grenoble, France where he searched for physics beyond the Standard Model in high-mass diphoton events at the LHC. Before joining the University of Washington, Buat was a research fellow at CERN and held postdoctoral fellowships at Simon Fraser University and Stony Brook. From 2017 to 2022, he led the ATLAS group studying Higgs boson couplings to leptons and the group responsible for the detection and characterization of hadronically-decaying tau leptons. His current research on ATLAS focuses on measurements of the Higgs and Z bosons in their decay to tau leptons. In 2022, Buat joined the PIONEER collaboration and is contributing to the prototyping of the experiment which is planned to start data-taking in 2030.