Improved search for two body muon decay ${mu}^+{rightarrow}e^+X_H$

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 ${mu}^+{rightarrow}e^+{ u}bar{ u}$ 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 ${Gamma}({mu}^+{rightarrow}e^+X_H)/{Gamma}({mu}^+{rightarrow}e^+{ u}bar{ u})$ at the level of $10^{-5}$ for this region.