Speaker
Description
The proton radius puzzle arises from the discrepancy between measurements of the proton charge radius obtained from muonic hydrogen spectroscopy and electron-based measurements, including electron-proton scattering and ordinary hydrogen spectroscopy. In 2010, the muonic hydrogen spectroscopy measurement reported a significantly smaller proton charge radius, differing by about 5σ from the CODATA value at the time. The Muon Scattering Experiment (MUSE) at the Paul Scherrer Institute (PSI) was designed to investigate this puzzle through simultaneous measurements of elastic electron-proton (ep) and muon-proton (μp) scattering from a liquid hydrogen target. MUSE uses the πM1 mixed secondary beam of electrons, muons, and pions at PSI, with beam momenta of 115, 160, and 210 MeV/c. Direct comparison of ep and μp cross sections provides a test of lepton universality in low Q² elastic scattering. MUSE also uses both positive and negative lepton beams to measure two-photon exchange effects, which can influence form factor and proton charge radius extraction. MUSE covers a Q² range of 0.0016–0.0820 GeV² for electrons and 0.0016–0.0799 GeV² for muons, providing sensitivity to the proton charge radius. This presentation will provide an overview of the experimental setup, current status, and recent progress of MUSE.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-SC0016577.