Recherche d’un neutrino lourd avec le détecteur ATLAS au LHC

This M.Sc. thesis describes a search for fourth generation heavy leptons using data from the ATLAS detector at LHC. The total integrated luminosity is 1.02 fb-1 in pp collisions at sqrt(s) = 7 TeV. This analysis study the single production of fourth generation neutral heavy lepton (N) via the charged channel and where W decays leptonically : pp -> W -> Ne -> eWe -> eevl (l = e ou ), which depends on the mixing element between the heavy lepton and the light lepton. The model of fourth generation leptons is constructed using FeynRules while the production of events is done by MadGraph 5.1.2.4. As point of reference, we chose a mass of 100 GeV for the fourth generation neutral heavy lepton (N) with $\xi^2_{N_e}$ = 0.19, which produce a cross section of 0.312 pb. The generation of the signal was done privately in Montreal and not by the ATLAS collaboration. Therefore the results cannot be considered official. With the simulation, the expected superior limit at 95% C.L. on the cross section is 0.145 pb with 0.294 pb for and 0.519 pb for 2. The expected superior limit at 95% C.L. on $\xi^2_{N_e}$ is 0.09.