Axionlike particles (ALPs) with lepton-flavor-violating couplings can be probed in exotic muon and tau decays. The sensitivity of different experiments depends strongly on the ALP mass and its couplings to leptons and photons. For ALPs that can be resonantly produced, the sensitivity of three-body decays such as $\mu \to 3e$ and $\tau \to 3\mu$ exceeds by many orders of magnitude that of radiative decays like $\mu \to e\gamma$ and $\tau \to \mu \gamma$. Searches for these two types of processes are therefore highly complementary. We discuss experimental constraints on ALPs with a single dominant lepton-flavor-violating coupling. Allowing for one or more such couplings offers qualitatively new ways to explain the anomalies related to the magnetic moments of the muon or the electron. The explanation of both anomalies requires lepton-flavor-nonuniversal or lepton-flavor-violating ALP couplings.

• Received 7 August 2019
• Accepted 30 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.211803

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Published by the American Physical Society