Constraining off-shell effects using low-energy Compton scattering

TL;DR

This paper uses low-energy Compton scattering data to tightly constrain off-shell effects in proton vertices, showing they are too small to explain the proton radius discrepancy in muonic hydrogen.

Contribution

It provides quantitative constraints on off-shell effects from experimental polarizability measurements, ruling out their significant role in the proton radius puzzle.

Findings

Off-shell effects are constrained to be two orders of magnitude smaller than needed.

Low-energy Compton scattering effectively limits off-shell contributions.

Off-shell effects cannot account for the muonic hydrogen Lamb shift discrepancy.

Abstract

Off-shell effects in proton electromagnetic vertices can be constrained from their effects on known processes. In particular, parameters in models for the off-shell effects can be determined by fitting to the proton electric and magnetic polarizabilities measured in low-energy Compton scattering. There has been recent speculation that off-shell effects contribute enough energy to the muonic hydrogen Lamb shift to explain the discrepancy between muonic and electronic measurements of the proton radius. We find that the constraints discussed here make the off-shell effects about two orders of magnitude smaller than needed for this purpose.