Search for a Non-Relativistic Boson in Two-Body Antimuon Decay

We demonstrate the feasibility of probing the charged lepton flavor violating decay ${\mu }^{+}\text{\hspace{0.17em}⁣}\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{e}^{+}{X}^0$ for the presence of a slow-moving neutral boson $X^0$ capable of undergoing gravitational binding to large structures, and as such able to participate in some cosmological scenarios. A short exposure to surface antimuons from beamline M20 at TRIUMF generates a branching ratio limit of $\lesssim {10}^{-5}$. This is comparable or better than previous searches for this channel, although in a thus-far unexplored region of $X^0$ phase space very close to the kinematic limit of the decay. The future improved sensitivity of the method using a customized p-type point contact germanium detector is described.