Elastic Positron-Proton Scattering at Low Q$^2$

TL;DR

This paper discusses the potential for high-precision elastic positron-proton scattering measurements at low Q$^2$ to resolve discrepancies in proton charge radius determinations and explore possible new physics.

Contribution

Proposes a new high-precision positron scattering experiment at Jefferson Lab to compare with electron data and address the proton radius puzzle.

Findings

Potential to clarify proton charge radius discrepancy

Extension to deuteron radius measurements

Constraints on radiative correction calculations

Abstract

Systematic differences in the the proton's charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The proton's charge radius, defined as the slope of the charge form factor at Q$^2$=0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.