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. Except for the case of tan$\beta$ $\approx$ 1, $|\mu| \sim 0$, %$M_2 \sim 0$, the lower limit on the chargino mass at 95\ confidence level for a heavy sneutrino For a sneutrino with mass above 300~GeV/$c^2$ and a mass difference between the chargino and the lightest neutralino above 10~GeV/$c^2$, the lower limit at 95\ confidence level on the chargino mass ranges from 84.3~GeV/$c^2$ to the kinematical limit (86.0~GeV/$c^2$), depending on the mixing parameters. The limit decreases for lower chargino-neutralino mass differences. The limit in the case of a light sneutrino is 67.6~GeV/$c^2$, provided that the second lightest chargino has a mass above 86~GeV/$c^2$, that the second lightest neutralino is heavier than the lightest chargino, and that there is no light sneutrino with a mass within 10~GeV$/c^2$ below the chargino mass. In the case of tan$\beta$ $\approx$ 1, $|\mu| \sim 0$, $M_2 \sim 0$ the limit on the chargino mass is 84 GeV/$c^2$ for large $m_0$. Upper limits on neutralino pair production cross-sections of about a picobarn are derived. The $(\mu,M_2)$ domain excluded in the MSSM-GUT scenario is determined by combining the neutralino and chargino searches. Compared to the previously published analyses, 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/$c^2$ for tan$\beta$ $\ge$ 1. The search has also been extended to the case where the lightest neutralino is unstable and decays into a photon and a gravitino.