Muonic bound systems, virtual particles and proton radius

TL;DR

This paper reviews theoretical models involving virtual particles and Lorentz violation to explain the proton radius puzzle, highlighting the use of highly charged ions and muonic systems as probes and discussing potential explanations and the need for further experiments.

Contribution

It summarizes current virtual particle and Lorentz-violating models addressing the proton radius discrepancy and discusses experimental probes and possible explanations.

Findings

Virtual particle models and Lorentz-violating theories are considered for the proton radius puzzle.

Muonic systems experience extremely strong electromagnetic fields comparable to high-Z ions.

Effective interactions and nonresonant effects may explain the discrepancy.

Abstract

The proton radius puzzle questions the self-consistency of theory and experiment in light muonic and electronic bound systems. Here, we summarize the current status of virtual particle models as well as Lorentz-violating models that have been proposed in order to explain the discrepancy. Highly charged one-electron ions and muonic bound systems have been used as probes of the strongest electromagnetic fields achievable in the laboratory. The average electric field seen by a muon orbiting a proton is comparable to hydrogenlike Uranium and, notably, larger than the electric field in the most advanced strong-laser facilities. Effective interactions due to virtual annihilation inside the proton (lepton pairs) and process-dependent corrections (nonresonant effects) are discussed as possible explanations of the proton size puzzle. The need for more experimental data on related transitions is emphasized.