Searching for New Physics at Μ→ E Facilities with Μ^+ and Π^+ Decays at Rest

We investigate the ability of mu -> e facilities, Mu2e and COMET, to probe, or discover, new physics with their detector validation datasets. The validation of the detector response may be performed using a dedicated run with mu+, collecting data below the Michel edge, Ee less than or similar to 52 MeV; an alternative strategy using pi+ -> e+nu e may also be considered. We focus primarily on a search for a monoenergetic e+ produced via two-body decays mu+ -> e+X or pi+ -> e+X, with X a light new physics particle. Mu2e can potentially explore new parameter space beyond present astrophysical and laboratory constraints for a set of well motivated models including axionlike particles with flavor violating couplings (mu+ -> e+a), massive Z' bosons (mu+ -> Z'e+), and heavy neutral leptons (pi+ -> e+N). The projected sensitivities presented herein can be achieved in a matter of days.