Searching for new physics at μ→e facilities with μ+ and π+ decays at rest

Abstract

We investigate the ability of μ→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 μ+, collecting data below the Michel edge, Ee≲52 MeV; an alternative strategy using π+→e+νe may also be considered. We focus primarily on a search for a monoenergetic e+ produced via two-body decays μ+→e+X or π+→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 (μ+→e+a), massive Z′ bosons (μ+→Z′e+), and heavy neutral leptons (π+→e+N). The projected sensitivities presented herein can be achieved in a matter of days. Published by the American Physical Society 2024