Búsqueda del Bosón de Higgs del Modelo Standard en el canal de desintegración H → WW∗ → 2μ2ν en el experimento CMS del LHC

A prospective analysis for the search of a SM Higgs boson in the Compact Muon Solenoid (CMS) experiment at the LHC is performed in this thesis, focusing in the gg → H → WW∗ → μνμν process. The search channel has a clear signature characterized by the presence of two high PT muons in the final state and a significant amount of missing ET due to the undetected neutrinos. No hard jet activity is expected. The principal sources of background come from several SM processes that may have similar final states as those of the signal, mainly W + jets, Z + jets, tt and the almost irreducible WW. Other sources of background are WZ, ZZ and tW processes. Final states coming from QCD processes have different signatures but very large cross-sections and they have also been studied. Several analysis techniques have been developed in this work. The main one is based on the application of a sequential selection criteria using a set of physical observables which characterize the main kinematic properties of the signal and discriminate with respect to the different backgrounds. Given the presence of neutrinos in the final state, that prevents the reconstruction of a clear Higgs mass peak, and the fact that the production cross-section for the Higgs signal is various orders of magnitude smaller than that of the backgrounds, the study relies in the control of the dominant sources of background. The analysis has been designed to effective ly reduced the different types of background processes. The muon reconstruction, identification and selection is optimized to select events with two muons coming from primary decays of W bosons and is the key to reject W + jets events when the W decays into a muon. A detailed data-driven muon fake rate study has been designed to treat this background as it has a large cross-section and can be a source of background when a second muon is wrongly identified. The hadronic activity present in the tt ̄ events has to be controlled, and events with hard jets coming from b decays have to be vetoed. The proper estimation of the missing ET in events with no genuine missing ET is crucial to reduce the Z + jets background to an acceptable level. The main variable to discriminate the signal from the WW background is the angle between the muons in the transverse plane to the beam, ∆φμμ. The muons emitted by the W bosons coming from the decay of a Higgs scalar are close in the transverse plane due to spin correlations, while the angular separation of the muons coming from W W decays tend to be larger. This is the most characteristic feature of the signal, and plays a major role in the discovery potential of the Higgs using H → W W ∗ process. The analysis developed in this thesis has been applied to different running scenarios and adapted to the evolving LHC running conditions. Initially, the design energy of the LHC of 14 TeV was co nsidered, and the study was performed assuming an integrated luminosity of 1 fb−1. Later the search was optimized, first to a centre of mass energy of 10 TeV and to an integrated luminosity of 200 pb−1 and finally to 7 TeV, the current centre of mass energy at which the LHC is running. The study at 7 TeV is done for an integrated luminosity of 1fb−1, that corresponds to the total data expected to be collected by the end of 2011.