Recherche des squarks quasi dégénérés avec la LSP dans l'expérience DELPHI au LEP

The present work deals with the search of third generation squarks in the framework of the MSSM with R-parity conservation, for small mass differences between these squarks and the LSP. Data collected at centre-of-mass energies front 189 to 209 GeV by DELPHI detector, corresponding to an integrated luminosity of 608 pb-1, are analysed in a search for three topologies. No evidence for deviations from the, Standard Model expectation is found. For a short lifetime stop a lower limit of 71 GeV/c2 is set for the, mass, independent of the mixing angle. Long-lived stop and sbottom of 97 GeV/c2 and 89 GeV/c2, respectivcly, arc also excluded. For Delta m > 5 GeV/c2 an intermediate lifetime stop with m < 52 GeV/c2 is excluded for all mixing angles; for Delta m > 2 GeV/c2, the, limit is set to 54 GeV/c2 for a loft-handed stop only.