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The $\mu^+ \to e^+ X$ decay, where $X$ is a dark sector boson, provides oneof the strongest available bounds on the scale of dark sector interactions. The$X$ boson can be an axion or a dark photon. We show that the concurrentdetermination of the anti-muon and positron polarizations makes possible todistinguish with a confidence level of 99% between the two dark sector portalswith as few as 6 observed events in the case of the massless dark photon.Instead, the massive spin-1, dimension 4 dark portal cannot be distinguishedfrom the axion-like case. We also discuss the possibility that the $X$ boson bea massive spin-2 particle.

Searching for the dark sector in two-body anti-muon decay with the polarization of monochromatic positrons