Muon elastic scattering with MUSE at PSI

The proton radius puzzle is the disagreement between the much more precise value determined from muonic hydrogen spectroscopy and the numerous atomic hydrogen and electron scattering determinations. The Muon Scattering Experiment (MUSE) recently approved at the Paul Scherrer Institute (PSI) has been designed to help resolve the puzzle by measuring the radius in a way not yet done. Similar to electron scattering, the radius will be extracted from the observed change of the charge form factor with momentum transfer. The experiment uses the πM1 beamline to provide a mixed secondary muon and electron (and pion) beam of either positive or negative charge. The comparison of muon and electron scattering measured simultaneously determines the consistency of the form factors in the two cases with high precision. Comparison of yields from both charge signs will at the same time disentangle the effect of two-photon exchange. It will be possible to extract the value of the proton charge radius from each set of the scattering data. (author)