Proton size anomaly

TL;DR

The paper investigates whether new exotic particles could explain the proton size discrepancy observed in muonic hydrogen, but finds that existing constraints largely disfavor such explanations.

Contribution

It systematically analyzes various hypothetical particles and interactions to assess their viability in explaining the proton size anomaly.

Findings

Many constraints from low energy data disfavor new particles as explanation.

Scalar, pseudoscalar, vector, and tensor interactions are considered.

Exotic particles coupling to muons are unlikely to resolve the discrepancy.

Abstract

A measurement of the Lamb shift in muonic hydrogen yields a charge radius of the proton that is smaller than the CODATA value by about 5 standard deviations. We explore the possibility that new scalar, pseudoscalar, vector, and tensor flavor-conserving nonuniversal interactions may be responsible for the discrepancy. We consider exotic particles that among leptons, couple preferentially to muons, and mediate an attractive nucleon-muon interaction. We find that the many constraints from low energy data disfavor new spin-0, spin-1 and spin-2 particles as an explanation.