Isotope dependence of muon-to-electron conversion

TL;DR

This paper analyzes how the rate of muon-to-electron conversion varies with different isotopes, aiding future experiments in selecting optimal targets to identify new physics beyond the Standard Model.

Contribution

It provides comprehensive conversion rate calculations for all stable isotopes and estimates nuclear-structure uncertainties to guide experimental target selection.

Findings

Conversion rates vary significantly across isotopes.

Nuclear-structure uncertainties are quantified.

Results support the use of mixed or enriched targets for new physics detection.

Abstract

The lepton-flavor-violating conversion of a muon into an electron in the field of a nucleus is one of the most sensitive probes of physics beyond the Standard Model and the experiments Mu2e, COMET, and DeeMe will explore uncharted terrain in the near future. An observation of this $\mu^-\to e^-$ conversion process opens up the possibility to distinguish the underlying operator or new-physics model by exploiting the target-nucleus dependence of the conversion rate. To facilitate the choice of optimal targets we provide $\mu^-\to e^-$ conversion rates for all stable isotopes and estimate nuclear-structure uncertainties. Our results enable studies of mixed or enriched target materials that are particularly promising for distinguishing scenarios in which the muon converts either on protons or neutrons.