The MUSE experiment aims to provide key missing information in the quest to resolve the proton radius puzzle: the difference between the 0.88 fm proton radius measured in atomic hydrogen and ep elastic scattering experiments and the 0.84 fm radius measured in muonic hydrogen. Over the last three years since the puzzle emerged, none of the many attempts to find a solution have reached a universally accepted resolution. Possible solutions include novel beyond standard model physics, novel hadronic physics and issues and/or underestimated uncertainties in the ep scattering data.
Recently there have been high precision electron scattering experiments at Mainz and JLab, and further measurements are planned. A series of excitation spectrum measurements on light nuclei are planned at PSI. The MUSE experiment will open up a new line of experimental investigations by measuring elastic muon scattering on the proton at the PSI piM1 beamline.
MUSE will scatter a mixed muon/pion/electron beam on a liquid hydrogen target with a Q^2 range of approximately 0.002 - 0.08 GeV2. Measurements of both μ+ and μ- at multiple beam momenta will enable checks of systematics, determination of two-photon exchange effects and magnetic contributions, and a radius determination at a similar level of precision to existing ep scattering experiments. Simultaneous electron scattering will allow similar tests and a direct comparison of the two probes, providing an excellent test of lepton universality. The physics background, status, and plans for the experiment will be discussed.