  We show that dark photons (A') with masses ~10-80 MeV can be probed in the
decay $\mu^+ \to e^+ \nu_e \bar{\nu}_\mu A', A' \to e^+ e^-$, with the upcoming
Mu3e experiment at the Paul Scherrer Institute (PSI) in Switzerland. With an
expected 10^{15} (5.5 x 10^{16}) muon decays in 2015-2016 (2018 and beyond),
Mu3e has the exciting opportunity to probe a substantial fraction of currently
unexplored dark photon parameter space, probing kinetic-mixing parameter,
epsilon, as low as epsilon^2 ~ 10^{-7} (10^{-8}). No modifications of the
existing Mu3e setup are required.

High Energy Physics - Phenomenology;High Energy Physics - Experiment