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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo>∼</a:mo> <a:mo stretchy="false">(</a:mo> <a:mn>115</a:mn> <a:mo>,</a:mo> <a:mn>280</a:mn> <a:mo stretchy="false">)</a:mo> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>MeV</a:mi> </a:math> , probing the ALP-electron-muon coupling down to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mo>∼</e:mo> <e:mn>4</e:mn> <e:mo>×</e:mo> <e:msup> <e:mn>10</e:mn> <e:mrow> <e:mo>−</e:mo> <e:mn>10</e:mn> </e:mrow> </e:msup> </e:math> for an ALP mass of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mo>∼</g:mo> <g:mn>200</g:mn> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:mi>MeV</g:mi> </g:math> .