Limits on new-lepton pairs (L−,L0) with arbitrary neutrino mass

We have searched 205 ${\mathrm{pb}}^{\mathrm{-}1}$ of √s =29 GeV data from the Mark II detector at the SLAC $e^{+}$ $e^{\mathrm{-}}$ storage ring PEP for events which may signify the existence of a new lepton pair ($L^{\mathrm{-}}$,$L^0$) where the $L^0$ may be massive, but does not exceed the $L^{\mathrm{-}}$ mass. Three event signatures for $L^{+}$ $L^{\mathrm{-}}$ decay are examined: (i) $e^{\pm }$ ${\mu }^{\mp }$, (ii) (e or μ${)}^{\pm }$(${\pi }^{\mp }$+≤4γ), and (iii) (e or μ${)}^{\pm }$(≥3${\pi }^{\pm }$+≥0γ${)}^{\mp }$. The numbers of signature events are found to be in good agreement with Monte Carlo simulations of known background sources. The ranges of $L^{\mathrm{-}}$ and $L^0$ masses for which new lepton pairs are excluded are obtained using a likelihood-ratio method to compare the number of observed signature events to the expected background, and to the expected background plus Monte Carlo predictions of ($L^{\mathrm{-}}$,$L^0$) signals.