Technical Design Report for the Paul Scherrer Institute Experiment R-12-01.1: Studying the Proton "Radius" Puzzle with μp Elastic Scattering

Abstract

The difference in proton radii measured with $\mu p$ atoms and with $ep$ atoms and scattering remains an unexplained puzzle. The PSI MUSE proposal is to measure $\mu p$ and $ep$ scattering in the same experiment at the same time. The experiment will determine cross sections, two-photon effects, form factors, and radii independently for the two reactions, and will allow $\mu p$ and $ep$ results to be compared with reduced systematic uncertainties. These data should provide the best test of lepton universality in a scattering experiment to date, about an order of magnitude improvement over previous tests. Measuring scattering with both particle polarities will allow a test of two-photon exchange at the sub-percent level, about a factor of four improvement on uncertainties and over an order of magnitude more data points than previous low momentum transfer determinations, and similar to the current generation of higher momentum transfer electron experiments. The experiment has the potential to demonstrate whether the $\mu p$ and $ep$ interactions are consistent or different, and whether any difference results from novel physics or two-photon exchange. The uncertainties are such that if the discrepancy is real it should be confirmed with $\approx$5$\sigma$ significance, similar to that already established between the regular and muonic hydrogen Lamb shift.

Cited by in corpus (7)

• The proton charge radius
• QED at NNLO with McMule
• The proton radius (puzzle?) and its relatives
• Operation and characterization of a windowless gas jet target in high-intensity electron beams
• Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering
• Characterization of Muon and Electron Beams in the Paul Scherrer Institute PiM1 Channel for the MUSE Experiment
• Ground state hyperfine structure of light muon-electron ions