We have searched for the annihilation of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ into the exclusive channels ${\mathit{e}}^{\pm }$${\mathrm{\tau }}^{\mp }$ and ${\mathrm{\mu }}^{\pm }$${\mathrm{\tau }}^{\mp }$ at √s =29 GeV, using 226 and 133 ${\mathrm{pb}}^{\mathrm{-}1}$, respectively, of data taken with the Mark II detector at the SLAC storage ring PEP. The resulting candidate sample is compatible with the expected background from τ pair production. Our analysis yields 95%-C.L. cross-section limits of ${\mathrm{\sigma }}_{\mathit{e}\mathrm{\tau }}$/${\mathrm{\sigma }}_{\mathrm{\mu }\mathrm{\mu }}$<1.8×${10}^{\mathrm{-}3}$ and ${\mathrm{\sigma }}_{\mathrm{\mu }\mathrm{\tau }}$/${\mathrm{\sigma }}_{\mathrm{\mu }\mathrm{\mu }}$<6.1×${10}^{\mathrm{-}3}$, where ${\mathrm{\sigma }}_{\mathrm{\mu }\mathrm{\mu }}$ is the QED cross section for production of a lepton pair. This is the first high-${\mathit{Q}}^2$ test of lepton-flavor conservation involving τ leptons.

• Received 6 April 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.1007