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

The mu+ --> e+ + X decay, where X is a dark sector boson, provides one of 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 concurrent determination of the antimuon and positron polarizations makes it possible to distinguish with a confidence level of 99% between the two dark sector portals with 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 distinguished from the axionlike case. We also discuss the possibility that the X boson be a massive spin-2 particle.