Search for muon-electron and muon-positron conversion

Limits on the lepton-flavor-violating reactions ${\mu }^{\mathrm{-}}$+Z→$e^{\mathrm{-}}$+Z and ${\mu }^{\mathrm{-}}$+Z→$e^{+}$+(Z-2), muon-electron and muon-positron conversion, have been obtained from a search performed at TRIUMF using a time-projection chamber. Upper limits (90% C.L.) for the branching ratios compared to ordinary muon capture for a titanium target are R—(Ti)=Γ(${\mathrm{\mu }}^{\mathrm{-}}$Ti→${\mathit{e}}^{\mathrm{-}}$Ti)/Γ(${\mathrm{\mu }}^{\mathrm{-}}$Ti capture)${10}^{\mathrm{-}12}$ and ${\mathit{R}}_{+}$(Ti)=Γ(${\mathrm{\mu }}^{\mathrm{-}}$Ti→${\mathit{e}}^{+}$Ca*)/Γ(${\mathrm{\mu }}^{\mathrm{-}}$Ti capture)<.1.7 class="aps-inline-formula">${10}^{\mathrm{-}10}$ A smaller data set obtained using a lead target yielded $R_{\mathrm{-}}$(Pb)${10}^{\mathrm{-}10}$. The implications of these results for extensions of the standard model which allow lepton-flavor violation are discussed.