Session 2WBB: Precision Muon and Pion Physics to Test the Standard Model II

The measurements of muon g-2, EDM, and muonium at J-PARC

The muon g−2 experiment at J-PARC is under preparation and targeted to measure the muon anomalous magnetic moment with the precision of 450 ppb and muon electric dipole moment with 1.5 × 10−21 e cm at its first stage. In addition, closely related measurements of muonium (a bound state of a positively charged muon and an electron), is in progress, such as 1s-2s and hyperfine splitting. These measurements will give us the improved value of muon mass and magnetic moment, which is very important parameters to the muon g−2 experiment.

I will briefly introduce the present status and future prospect of these projects.   

The Latest Results from the CLFV Search Experiment MEG II

The MEG II experiment searches for the charged lepton flavour violating (CLFV) decay, μ→eγ with a sensitivity goal enhanced by one order of magnitude compared to the final sensitivity of the previous MEG experiment, based on upgraded detectors with improved performance. The MEG II has started a physics data-taking in year 2022 after a pilot-run in year 2021. The latest results on the from the μ→eγ search in the MEG II experiment will be reported.   

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.