The Effective Theory of Muon-to-Electron Conversion

TL;DR

This paper develops an effective low-energy theory for muon-to-electron conversion, enabling the analysis of experimental data to identify the underlying particle physics operators responsible for charged lepton flavor violation.

Contribution

It introduces a systematic expansion-based effective theory that separates particle and nuclear physics contributions, and highlights the potential of inelastic nuclear transitions to identify CLFV operators.

Findings

Inelastic transitions to low-energy nuclear states can be used to identify CLFV operators.

Experiments like Mu2e and COMET can exploit nuclear responses to discover CLFV.

Theoretical framework connects low-energy results with high-energy physics constraints.

Abstract

We summarize recent work to develop an effective theory of muon-to-electron conversion, based on a complete set of low-energy effective operators that are developed from a systematic expansion in velocities and momenta. The expansion effectively factors rates into sums of particle physics and nuclear physics terms, where the former are expressed as bilinears in the LECs (the low-energy constants of the effective theory) and the latter are the associated nuclear responses. One can view the nuclear responses as ``dials" that can be adjusted -- for example, by selection of targets with specific properties -- in order to isolate the former. We show that an important dial, in the case of Mu2e and COMET, will be inelastic transitions to certain low-energy nuclear states that are resolvable in 27Al. If these transitions are exploited, the experiments have the potential not only to discover charged lepton flavor violation (CLFV), but to determine the operators responsible for the CLFV. We also discuss how such low-energy results can be ``ported" to higher energies through a tower of matched EFTs, so they can be combined with other experimental limits to further constrain CLFV