  Experiments looking for a lepton flavor-violating decay
$\mu^{+}\!\!\rightarrow \!e^{+} X^{0}$ 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.
