Search for charginos , neutralinos , and gravitinos at LEP

An update of the searches for charginos and neutralinos in DELPHI is presented, based mainly on recent data collected at centre-of-mass energies of 161 GeV and 172 GeV. No signal is found. For a sneutrino with mass above 300 GeV/c and a mass difference between the chargino and the lightest neutralino above 10 GeV/c the lower limit at 95% confidence level on the chargino mass ranges from 84.5 GeV/c to the kinematical limit (86.0 GeV/c), depending on the mixing parameters. This limit decreases for lower mass differences. The limit in the case of a light sneutrino is 67.8 GeV/c. Upper limits on neutralino pair production cross-sections of about a picobarn are also derived. The (μ,M2) domain excluded in the MSSM-GUT scenario is determined by combining the neutralino and chargino searches. These results imply a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 24.9 GeV/c for tanβ ≥ 1. The search has also been extended to the case where the lightest neutralino is unstable and decays into a photon and a gravitino.