Etude de l'identification de leptons tau pour la recherche de supersymétrie dans le cadre de l'expérience CMS au LHC

The work presented in this thesis deals with tau identified by different algorithms in pp collision at LHC and in various kind of physics processes : Electroweak, top quark production, SUSY and QCD. Tau leptons are geometrically identified in their hadronic decay mode because the jets coming from those decays are generally more collimated and isolated than QCD jets. An additional discrimination is applied in asking the presence of one of the dominant # decay modes containing one or three charged pions and up to two neutral pions. The QCD jets produced in large quantities in LHC proton-proton collisions constitute a large background source for analyses involving tau-ID. The analysis of QCD jets produced in LHC for fake-rate studies processed 36pb−1 of data registered during year 2010. Identification efficiencies and fake-rate distributions reveals a strong dependency on event type : distributions are globally lower in events having a large hadronic activity like SUSY and tt events than in electroweak and QCD events. The environment effects of the collision on the identification of tau leptons can be parameterized by two global variables : somme(ET) and charged hadron multiplicity. Efficiencies drop down by 80% between the lowest and the highest charged hadron multiplicities.