Optimisation de l'herméticité du détecteur DELPHI pour la recherche de particules supersymétriques à LEP2

In new particles searches at LEP, for the ones characterised by missing energy, it is very important to have the best hermeticity in detecting particles in final states. Photon counters were installed during 1994 and 1995 to cover the weak region in photon detection, between the central part and the end-caps of DELPHI. We discuss the working principles and the tests done on the counters. The optimisation of the counters and of the detectors already present in the other weak regions of DELPHI was examined. We estimate that the efficiency of detecting photons over all the useful solid angle changes from 97\% to more than 99.6\%. This was verified using the 1994 leptonic data of LEP at 95\% confidence level. A search for the lightest supersymmetric partners of the charged bosons (charginos) was done at LEP1.5. All the hermeticity detectors were used to reduce at minimum the contamination from radiative backgrounds. The theoretical motivations and the phenome\-nological de\-scription of the chargiNO1are given. The search was done on the data collected at LEP in November 1995 at 130.4 and 136.3 GeV. We put different lower limits on the chargino mass which depend on the parameters of the theoretical model. They vary between 56.3~GeV and 66.8 GeV. One chargiNO1candidate was found in the data, that is compatible with the prediction from background estimation.