Technical Design Report for the Paul Scherrer Institute Experiment R-12-01.1: Studying the Proton "Radius" Puzzle with {\mu}p Elastic Scattering

Understand

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 $e p$ 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.