Studying the Proton "Radius" Puzzle with \mu p Elastic Scattering

TL;DR

This paper discusses the MUSE experiment's approach to resolving the Proton Radius Puzzle by comparing muon-proton and electron-proton elastic scattering, aiming to clarify discrepancies in proton radius measurements.

Contribution

It introduces the MUSE experiment, which simultaneously measures muon and electron scattering to test various hypotheses about the proton radius discrepancy.

Findings

Expected measurement precision of ~0.01 fm for the proton radius.

Tests of beyond-standard-model physics and nucleon structure models.

Assessment of systematic uncertainties in scattering experiments.

Abstract

The Proton Radius Puzzle is the inconsistency between the proton radius determined from muonic hydrogen and the proton radius determined from atomic hydrogen level transitions and ep elastic scattering. No generally accepted resolution to the Puzzle has been found. Possible solutions generally fall into one of three categories: the two radii are different due to novel beyond-standard-model physics, the two radii are different due to novel aspects of nucleon structure, and the two radii are the same, but there are underestimated uncertainties or other issues in the ep experiments. The MUon proton Scattering Experiment (MUSE) at the Paul Scherrer Institut is a simultaneous measurement of \mu^+ p and e^+ p elastic scattering, as well as \mu^- p and e^- p elastic scattering, which will allow a determination of the consistency of the \mu p and the ep interactions. The differences between + and - charge scattering are sensitive to two-photon exchange effects, higher-order corrections to the scattering process. The slopes of the cross sections as Q^2 -> 0 determine the proton "radius". We plan to measure relative cross sections at a typical level of a few tenths of a percent, which should allow the proton radius to be determined at the level of ~0.01 fm, similar to previous ep measurements. The measurements will test several possible explanations of the proton radius puzzle, including some models of beyond-standard-model physics, some models of novel hadronic physics, and some issues in the radius extraction from scattering data.