Abstract
The proton radius puzzle is the difference between the proton radius as measured with electron scattering and in the excitation spectrum of atomic hydrogen, and that measured with muonic hydrogen spectroscopy. Since the inception of the proton radius puzzle in 2010 by the measurement of Pohl et al.[1], many possible resolutions to the puzzle have been postulated, but, to date, none has been generally accepted. New data are therefore necessary to resolve the issue. We briefly review the puzzle, the proposed solutions, and the new electron scattering and spectroscopy experiments planned and underway. We then introduce the MUSE experiment, which seeks to resolve the puzzle by simultaneously measuring elastic electron and muon scattering on the proton, in both charge states, thereby providing new information to the puzzle. MUSE addresses issues of two-photon effects, lepton universality and, possibly, new physics, while providing simultaneous form factor, and therefore radius, measurements with both muons and electrons.
Highlights
The proton radius puzzle (PRP) concerns the difference between the radius of the proton as measured with electron scattering and atomic hydrogen spectroscopy, and that measured in muonic hydrogen
Remaining explanations include possible physics beyond the Standard Model (BSM), whereby the electron is measuring an electromagnetic radius, while the muon is measuring an electromagnetic radius modified by BSM effects
Measure elastic muon scattering on the proton, simultaneously with elastic electron scattering in order to have a very firm grasp on possible systematic uncertainties
Summary
The proton radius puzzle (PRP) concerns the difference between the radius of the proton as measured with electron scattering and atomic hydrogen spectroscopy, and that measured in muonic hydrogen. In 2010, the CREMA Collaboration published their measurement of the proton radius Rp = 0.8409(4) fm, which was made by studying the Lamb shift in muonic hydrogen [1]. This result, ten times more precise than, is completely incompatible with the CODATA value of Rp = 0.8775(51) fm [2]. By measuring elastic scattering cross sections down to low Q2, it is possible to test for the new particles postulated as part of the BSM explanation By measuring in both charge states, MUSE can access two-photon exchange effects, postulated to be responsible for the breakdown of the Rosenbluth separation and which would be enhanced by the enhanced proton polarizability proposition
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