  Charged lepton flavor violating muon decay ${\mu}^+{\rightarrow}e^+X_H$,
where $X_H$ is a massive neutral boson, was sought by searching for extra peaks
in the muon decay ${\mu}^+{\rightarrow}e^+{\nu}\bar{\nu}$ energy spectrum in
the $m_{X_H}$ mass region $47.8-95.1$ MeV/$c^2$. No signal was found and 90%
confidence level upper limits were set on the branching ratio
${\Gamma}({\mu}^+{\rightarrow}e^+X_H)/{\Gamma}({\mu}^+{\rightarrow}e^+{\nu}\bar{\nu})$
at the level of $10^{-5}$ for this region.
