Update of the search for charginos nearly mass-degenerate with the lightest neutralino

The data collected by DELPHI in 1998 at the centre-of-mass energy of 189 GeV have been used to update the search for charginos nearly mass-degenerate with the lightest supersymmetric particle, which is assumed to be the lightest neutralino. Mass differences below ΔM=3 GeV/c2 are considered. No excess of events with respect to the Standard Model expectation has been observed, and exclusions in the plane of ΔM versus chargino mass are given. The new ΔM independent lower limit on the mass of the chargino is 62.4 GeV/c2 in the higgsino scenario (which includes the gaugino mass unification scenario), if all sfermions are heavier than the lightest chargino. In the approximation of large sfermion masses the limit is 59.8 GeV/c2, independently of the field content.