Lepton flavor violation from diphoton effective interactions

We consider charged lepton flavor violating transitions mediated by the diphoton effective interactions ℓiℓjγγ and explore which processes can probe them better. Our analysis includes single and double radiative decays, ℓi→ℓjγ(γ), as well as ℓi→ℓj conversions in nuclei for all possible flavor combinations, which we compute for the first time for ℓ→τ conversions in this framework. We find that currently the best limits are provided by the loop-induced ℓi→ℓjγ processes, while the best future sensitivities come from μ→e conversion in aluminum and from potential τ→ℓγγ searches at Belle II or at the Super Tau Charm Facility. We also motivate the search for μ→eγγ at the Mu3e experiment as a complementary probe of these operators.