Sensitivity of $\mu\to e$ processes to $\tau$ flavour change

International audienceTransforming a μ to a τ, then the τ to to an e, results in μ→e. In an effective field theory (EFT) framework, we explore the sensitivity of μ→e observables to products of (μ→τ)×(τ→e) interactions and show that the exceptional sensitivity of upcoming μ↔e experiments could allow us to probe parameter space beyond the reach of upcoming τ↔l searches in Higgs, τ, and B decays. We describe the τ↔l interactions as dimension six operators in the SM EFT, identify pairs of them giving interesting contributions to μ↔e processes, and obtain the anomalous dimensions mixing those pairs into dimension eight μ→e operators. We find that μ→e processes are sensitive to τ flavor-changing B decays at rates comparable to current B anomalies, but lepton flavour violating operators cannot reduce B rates—as appropriate in many current B anomalies—because they do not interfere with the SM