Search for anomalous production of acoplanar di-lepton events in e+e- collisions at √(s)=183 and 189 GeV

A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 237.4 pb-1 at e+e- centre-of-mass energies of approximately 183 GeV and 189 GeV. The observed numbers of events - 78 at 183 GeV and 301 at 189 GeV - are consistent with the numbers expected from standard model processes, which arise predominantly from W+W- production with both W bosons decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Discrimination techniques are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent. Upper limits on the production cross-section times branching ratio squared for sleptons and for leptonically decaying charginos and charged Higgs are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay l±R→l±χ01, where χ01 is the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 82.3 GeV for mμ--mχ01>3 GeV and right-handed staus with masses below 81.0 GeV for mτ--mχ01>8 GeV. Right-handed selectrons are excluded at 95% CL for masses below 87.1 GeV for me--mχ01>5 GeV, within the framework of the minimal supersymmetric standard model assuming μ<-100 GeV and tan β=1.5. Charged Higgs bosons, H±, are excluded at 95% CL for masses below 82.8 GeV, assuming a 100% branching ratio for the decay H±→τ±ντ. (orig.)