Flavor violating muon decay into an electron and a light gauge boson

We analyze the flavor violating muon decay μ→eχ, where χ is a massive gauge boson, with emphasis in the regime where χ is ultralight. We first study this process from an effective field theory standpoint in terms of form factors. We then present two explicit models where μ→eχ is generated at tree level and at the one-loop level. We also comment on the prospects of observing the process μ→eχ in view of the current limits on μ→3e from the SINDRUM collaboration.