Improved search for two body muon decay ${\mu }^{+}\to e^{+}{X}_H$

Charged lepton flavor violating muon decay ${\mu }^{+}\to e^{+}{X}_H$, where $X_H$ is a massive neutral boson, was sought by searching for extra peaks in the muon decay ${\mu }^{+}\to e^{+}\nu \bar{\nu }$ 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 $\mathrm{\Gamma }({\mu }^{+}\to e^{+}{X}_H)\mathrm{/}\mathrm{\Gamma }({\mu }^{+}\to e^{+}\nu \bar{\nu })$ at the level of ${10}^{-5}$ for this region.