A search for a cosmologically-relevant boson in muon decay

Experiments looking for a lepton flavor-violating decay ${\mu }^{+}\text{\hspace{0.17em}⁣}\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{e}^{+}{X}^0$ are reviewed in light of present-day germanium detector technology, with an eye on scenarios where a long-lived, slow-moving massive boson $X^0$ might have a cosmological impact. A broad swath of interesting, unexplored parameter space very close to the kinematic limit of the decay is found to be within the reach of a new proposed search. A number of possible roles for $X^0$ in past and present epochs can be investigated.