Search for acoplanar lepton pair events in e$^+$e$^-$ collisions at $\sqrt{s} = 161$, 172 and 183 GeV

Abstract.

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 ${\rm W^+W^-}$ 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 limits on the production cross-section times branching ratio squared for various new physics processes are presented in a manner intended to minimise the number of model assumptions. Assuming a 100% branching ratio for the decay $\tilde\ell^\pm_R \rightarrow \ell^\pm \tilde\chi^0_1$, we exclude at 95% CL: right-handed smuons with masses below 65 GeV for $m_{\tilde\mu^-} - m_{\tilde\chi^0_1} > 2$ GeV and right-handed staus with masses below 64 GeV for $m_{\tilde\tau^-} - m_{\tilde\chi^0_1} > 10$ GeV. Right-handed selectrons are excluded at 95% CL for masses below 77 GeV for $m_{\tilde{\rm e}^-} - m_{\tilde\chi^0_1} > 5$ GeV within the framework of the Minimal Supersymmetric Standard Model assuming $\mu < -100$ GeV and $\tan{\beta} = 1.5$.