Search for chargino and neutralino production at

Abstract

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of $10.3$ pb $^{-1}$ at centre-of-mass energies of $\sqrt{s} = $ 170 and 172 GeV with the OPAL detector at LEP. No evidence for these particles has been found. The results are combined with those from previous OPAL chargino and neutralino searches at lower energies to obtain limits. Exclusion regions at 95% C.L. of parameters of the Minimal Supersymmetric Standard Model are determined. Within this framework, for $\tan\beta \ge 1.0$ , lower mass limits are placed on the lightest chargino and the three lightest neutralinos. The 95% C.L. lower mass limit on the lightest chargino, assuming that it is heavier than the lightest neutralino by more than 10 GeV, is 84.5 GeV for the case of a large universal scalar mass ( $m_0 >$ 1 TeV) and 65.7 GeV for the smallest $m_0$ compatible with current limits on the sneutrino mass and slepton cross-sections. The lower limit on the lightest neutralino mass at 95% C.L. for $\tan \beta \geq 1.0$ is 24.7 GeV for $m_0 = 1$ TeV and 13.3 GeV for the minimum $m_0$ scenario. These mass limits are higher for increasing values of $\tan\beta$ . The interpretation of the limits in terms of gluino and scalar quark mass limits is also given.