Supernova constraints on lepton flavor violating axions

Supernovae offer a unique hot and dense environment to probe new physics beyond the Standard Model. We investigate supernova cooling constraints on lepton-flavor-violating (LFV) axions and axionlike particles (ALPs) that couple to electrons and muons. For LFV-ALP production in supernovae, muon decay and lepton bremsstrahlung have been considered previously. In this work, we identify the electron-muon coalescence channel as an efficient new production mechanism in the high-mass regime. We also include the semi-Compton scattering process, which has recently been shown to provide sizable contributions for electron-coupled ALPs. We find that muon decay dominates in the low-mass regime, electron-muon coalescence becomes the leading channel at high masses, and semi-Compton scattering provides the dominant contribution in the intermediate mass range. We find that the electron-muon coalescence process yields the strongest constraints in the mass range of ∼ ( 115 , 280 ) MeV , probing the ALP-electron-muon coupling down to ∼ 4 × 10 − 10 for an ALP mass of ∼ 200 MeV .