Evidence for Anomalous Lepton Production in e+ - e- Annihilation

We have found events of the form ${e}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ifmmode\pm\else\textpm\fi{}}+{\ensuremath{\mu}}^{\ensuremath{\mp}}+\mathrm{missing}\mathrm{energy}$, in which no other charged particles or photons are detected. Most of these events are detected at or above a center-of-mass energy of 4 GeV. The missing-energy and missing-momentum spectra require that at least two additional particles be produced in each event. We have no conventional explanation for these events.