Search for muon-electron and muon-positron conversion

Limits on the lepton-flavor-violating reactions μ^-+Z{→}e^-+Z and μ^-+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}(μ^-Ti{→}e^-Ti)/{Γ}(μ^-Ti capture)4.6×{}10^-12 and R₊(Ti)={Γ}(μ^-Ti{→}e⁺Ca*)/{Γ}(μ^-Ti capture).1.7×{}10^-10 A smaller data set obtained using a lead target yielded R_-(Pb)4.9×{}10^-10. The implications of these results for extensions of the standard model which allow lepton-flavor violation are discussed.