Search for the Reactionμ−+Cu→e++Co

We have searched for the reaction ${\mu }^{-}+\mathrm{Cu}\to e^{+}+\mathrm{Co}$, which is allowed by the lepton conservation scheme that assigns the same lepton number to $e^{-}$, ${\mu }^{+}$, ${\nu }_e$, and ${\overline{\nu }}_{\mu }$. One event was observed which was consistent with the above reaction, while the expected background was ≃ 0.02 events. Although we cannot be certain whether the process was detected, we can set an upper limit on the branching ratio relative to ordinary muon capture of $R<~2.6\times {10}^{-8\mathrm{}}$ (90% confidence level).