A submission to the 2020 update of the European Strategy for Particle Physics on behalf of the COMET, MEG, Mu2e and Mu3e collaborations

TL;DR

This paper reviews current and future muon-based charged-lepton flavor violation experiments, highlighting their potential to probe new physics models at very high energy scales and complement collider searches.

Contribution

It summarizes the status, capabilities, and future prospects of muon cLFV experiments at major facilities, emphasizing their role in exploring physics beyond the Standard Model.

Findings

Experiments aim for sensitivity to new physics scales up to 10^4 TeV/c^2.

Next-generation experiments could start within the next decade.

These experiments complement and extend LHC searches for new physics.

Abstract

Charged-lepton flavour-violating (cLFV) processes offer deep probes for new physics with discovery sensitivity to a broad array of new physics models - SUSY, Higgs Doublets, Extra Dimensions, and, particularly, models explaining the neutrino mass hierarchy and the matter-antimatter asymmetry of the universe via leptogenesis. The most sensitive probes of cLFV utilize high-intensity muon beams to search for $\mu \rightarrow e$ transitions. We summarize the status of muon-cLFV experiments currently under construction at PSI, Fermilab, and J-PARC. These experiments offer sensitivity to effective new physics mass scales approaching O($10^4$) TeV/c$^2$. Further improvements are possible and next-generation experiments, using upgraded accelerator facilities at PSI, Fermilab, and J-PARC, could begin data taking within the next decade. In the case of discoveries at the LHC, they could distinguish among alternative models; even in the absence of direct discoveries, they could establish new physics. These experiments both complement and extend the searches at the LHC.