Search for Acoplanar Lepton Pair Events in e+e- Collisions at sqrt{s} =   161, 172 and 183 GeV
  A selection of di-lepton events with significant missing transverse momentum has been performed using a total data sample of 77.0 pb-1 at e+e- centre-of-mass energies of 161 GeV, 172 GeV and 183 GeV. The observed numbers of events: four at 161 GeV, nine at 172 GeV, and 78 at 183 GeV, are consistent with the numbers expected from Standard Model processes, which arise predominantly from W+W- production with each W decaying leptonically. This topology is an experimental signature also for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is apparent and model independent limits on the production cross-section times branching ratio squared for various new physics processes are presented. Assuming a 100% branching ratio for the decay of a right-handed charged slepton to a charged lepton and the lightest neutralino, we exclude at 95% CL: right-handed smuons with masses below 65 GeV for smuon - neutralino mass differences greater than 2 GeV and right-handed staus with masses below 64 GeV for stau - neutralino mass differences greater than 10 GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for selectron - neutralino mass differences greater than 5 GeV within the framework of the Minimal Supersymmetric Standard Model assuming mu < -100 GeV and tan{beta} = 1.5. 
The OPAL Collaboration, G. Abbiendi et al