Experimental Searches for Muon to Electron Conversion in a Nucleus: COMET, DeeMe, and Mu2e. A Contributed paper for Snowmass 21

TL;DR

This paper reviews three upcoming experiments—COMET, DeeMe, and Mu2e—that aim to detect muon-to-electron conversion in nuclei, providing sensitive probes for new physics beyond the Standard Model, with potential to reach very high energy scales.

Contribution

It details the design, motivation, and expected timelines of three major experiments searching for muon-to-electron conversion, highlighting their potential to explore physics beyond current collider capabilities.

Findings

Experiments will probe new physics scales up to ~10^4 TeV.

Expected to improve sensitivity to charged lepton flavor violation.

Support for these experiments is strongly encouraged.

Abstract

Searches for charged lepton flavor violation (CLFV) offer deep probes for a range of new physics scenarios, such as super-symmetric models, theories involving scalar leptoquarks or additional Higgs doublets, and models explaining the neutrino mass hierarchy and the matter-antimatter asymmetry of the universe via leptogenesis. The coherent, neutrinoless conversion of a muon to an electron in the field of a nucleus, $\mu^{-}+N\rightarrow e^{-} +N$, is one example of a muonic CLFV process which has sensitivity to this new physics. This paper details three experiments: COMET, DeeMe and Mu2e which will search for $\mu^{-}+N\rightarrow e^{-} +N$ in the coming decade. These experiments offer sensitivity up to an effective new physics mass scale of $\mathcal{O}(10^{4}$ TeV/c$^{2}$), going far beyond what can be achieved in direct, collider-based, searches. The theoretical motivation, designs and anticipated timelines for these three experiments are presented. These experiments are a crucial part of a global search for CLFV. Continued support for all planned experimental searches for muonic CLFV is strongly encouraged.