Supernova constraints on lepton flavor violating ALPs

TL;DR

This paper explores how supernova observations can constrain lepton flavor violating axion-like particles, identifying new production mechanisms and providing the strongest bounds on their couplings in certain mass ranges.

Contribution

It introduces the electron-muon coalescence channel as a new dominant production mechanism for LFV-ALPs in supernovae, expanding previous models.

Findings

Electron-muon coalescence dominates at high ALP masses.

Semi-Compton scattering significantly contributes at intermediate masses.

Constraints on ALP-electron-muon coupling reach down to 4×10⁻¹⁰ at 200 MeV.

Abstract

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 axion-like 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 $\sim (115,280)$ MeV, probing the ALP-electron-muon coupling down to $\sim 4\times 10^{-10}$ for an ALP mass of $\sim200$ MeV.