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

Understand

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$^2$.