Abstract

Multihadronic ${e}^{+}{e}^{\ensuremath{-}}$ annihilation events containing final-state leptons have been investigated with a 4.7-${\mathrm{pb}}^{\ensuremath{-}1}$ data sample at c.m. energies of 50 and 52 GeV. The number of low-thrust events ($T\ensuremath{\le}0.8$) with isolated leptons is consistent with the expectation of the standard model with five quark flavors. Limits on the production of new quarks and leptons are reported.