A robust search for lepton flavour violating axions at Mu3e

A bstract We propose a search at Mu3e for lepton flavor violating axion(-like) particles in μ → 3 e + a decays. By requiring an additional e + e − pair from internal conversion, one can circumvent the calibration challenges which plague the μ → e + a channel for axions lighter than 20 MeV. Crucially, the corresponding reduction in signal rate is to a large extent compensated for by Mu3e ’s ability to resolve highly collimated tracks. For phase I of Mu3e , we project a sensitivity to decay constants as high as 6 × 10 9 GeV which probes uncharted parameter space in scenarios of axion dark matter. The sensitivity to axions which couple primarily to right-handed leptons can be further improved by leveraging the polarisation of the muon beam.