Speaker
Description
The nuclei-assisted transition $\mu^- e^- \to e^- e^-$ in a muonic atom is a striking charged-lepton flavor-violating process that muon-conversion experiments such as Mu2e and COMET will probe. We analyze this process in a simplified axion-like particle (ALP) model with couplings to light charged leptons and, possibly, to a dark sector, and compute the transition rate. The rate scales with the cube of the effective nuclear charge, and in contrast to heavy-mediator scenarios, light ALPs can parametrically enhance the branching ratio by lifting the propagator suppression. We confront the model with a broad set of low-energy, astrophysical, and beam-dump constraints on ALP-lepton interactions, and find that the maximal branching ratio in aluminum is $\sim \mathcal{O}(10^{-20})$, with even stronger suppression in the resonant ALP-mass region. Remarkably, the surviving parameter space lies close to the current $\mu \to 3e$ limit, implying that the upcoming Mu3e experiment will probe the most phenomenologically relevant region.