Probing μeγγ contact interactions with μ→e conversion

Contact interactions of a muon, an electron and two photons can contribute to the decay $\mu \to e \gamma \gamma$, but also to the conversion of a muon into an electron in the electric field of a nucleus. We calculate the $\mu \to e$ conversion rate, and show that for the coefficients of operators involving the combination $FF \propto |\vec{E}|^2$ (as opposed to $F\tilde{F} \propto \vec{E} \cdot \vec{B}$), the current bound on $\mu \to e$ conversion is more sensitive than the bound on $\mu \to e \gamma \gamma$. Constraining these $\bar{e}\mu FF$ operators gives the best sensitivity to some dimension six operators.