Status of the Muon Scattering Experiment (MUSE) at PSI

The proton radius puzzle was established in 2010 when the results of a reduced size of the proton obtained with muonic hydrogen spectroscopy were first released. Previously, measurements with regular hydrogen spectroscopy were consistent with low-energy electron scattering, both giving a larger proton radius. The primary motivation of the Muon Scattering Experiment (MUSE), now under construction at Paul-Scherrer Institute (PSI), is to investigate whether the difference seen between muonic and regular hydrogen spectroscopy is also present in comparison of muon and electron scattering from the proton. Different explanations of the puzzle would favor different scenarios for the outcome of MUSE. The puzzle could be restricted to the understanding of the bound state only, in which case scattering should be species-independent. On the other hand, if the discrepancy is due to a fundamental difference between $e$ and $\mu$ beyond the Standard Model, differences may be found in the scattering case, too. The MUSE experiment has been designed to measure the proton radius, proton form factors and lepton-proton elastic cross sections with competitive uncertainties to clearly distinguish between explanation scenarios. The use of electron and muon beams of either charge allows for four independent measurements, to directly probe the differences between lepton species, and to simultaneously disentangle effects due to two-photon exchange, which constitute some of the largest uncertainties in the theory framework not yet validated by experimental data. In this presentation, I will review the motivation for MUSE, and describe the experiment under construction, its present status and timeline for completion, and possible impact. 

On behalf of the MUSE Collaboration.