A search for lepton flavour violating Z⁰ decays

Abstract

We have searched for lepton flavour violating Z⁰→eμ, Z⁰→eτ and Z⁰→μτ decays in a sample of 4.0×10⁶ visible Z⁰ decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z⁰→eμ. The samples of selected Z⁰→eτ and Z⁰→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level:

$$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$