Direct search for Higgs boson in LHCb

The LHCb detector is a forward one-arm spectrometer to precision measurements of CP violation in the B-meson systems. The motivation of the present work is to assess the potential of LHCb to observe a Standard Model (SM) Higgs signal. The recent results obtained at LEP give a hint of a SM Higgs boson with a mass mH = 115.0 +1.3 &#8211;0.9 GeV/c2 with a statistical significance of 2.9 standard deviations. Because of the high longitudinal boost encountered by the products in the pp collisions at LHC, a significant fraction (~30%) of light Higgs (mH = 115 GeV/c2) are produced in the LHCb acceptance 1.8 < h < 4.9. These facts potentially place LHCb in the race for the observation of the SM Higgs. Given a relatively low running luminosity of 2 x 1032 cm-2s-1- compared to the nominal 1034 cm-2s-1 at LHC and a limited geometrical acceptance, we have shown that the channels accessible to LHCb are H + W± Z0 b`b + l± X for Higgs masses in the range 100-130 GeV/c2. This work pioneered a setup for the production and the analysis of hard jets in the LHCb detector. We demonstrated in the full detector simulation that the LHCb baseline design allows to efficiently identify, reconstruct and trigger the b-jets coming from the Higgs. Due to the impossibility to perform a detailed simulation of the huge amount of background, we have developed a "fast simulation" which includes the relevant detector effects. The Higgs analysis requires a hard lepton isolated from the b-jets to reject the QCD background. At this stage, the cut-based study indicates that the dominant background comes from top pairs production. The signal significance is S/sqrt(B) ~0.7 for one LHCb year (integrated luminosity of 2 fb-l). A realistic scheme for b-jets tagging and for the identification of the associated lepton is left for further studies. This work initiated an involved analysis that deserves to be continued. The signal significance is modest, however, potentially large improvements must not be neglected when performing a combined optimization on the discriminating variables. This thesis also suggests some new strategies to enhance the signal significance. Part of the work done for this thesis was the participation in technical developments for LHCb. Appendix C summarizes the activity led in the context of the RD46 collaboration at CERN during the years 1996-1998 to develop the capillary layers technique for tracking purpose in high luminosity environment. The various publications to which the present work contributed are listed at the end of the appendix.