Recherche directe à LHCb du boson de Higgs en production associée et contribution au développement du nouveau trajectographe à fibres scintillantes

The framework of this thesis is the Large Hadron collider, more precisely the LHCb experiment. The technical part of this work focusses on the upgrade project of the LHCb experiment, which is planned to be achieved in 2018. The LPHE is responsible for the development of the new Tracker, based on a scintillating fibers technology. A computer simulation of the fiber lattice is presented in order to estimate the efficiency and the resolution of this new detector device. These values are strongly dependent on the transverse $\theta$ angle of the incident particles. For small values of $\theta$ (from 0 to 15$^\circ$), the resolution is found to be around 50 $\mu m$ and the efficiency is close to 100 %. For $\theta$ in the range 15 to 25$^\circ$, the resolution is degrading to about 79 $\mu m$ but the efficiency remains stable around 100 %. When the angle reaches values from 25 to 35$^\circ$, the resolution falls down to 335 $\mu m$ and the efficiency is merely 50 %. These results have to be compared to the experimental measurements obtained by means of a detector prototype. The main part of this thesis details the measurement of the number of Higgs events produced in association with a vector boson ($Z^0$ or $W^\pm$) at a center of mass energy of 7 $TeV$. In 2011, the energy of the collisions produced by the LHC is 7 $TeV$, while the energy goes up to 8 $TeV$ in 2012. We are waiting for the year 2015 to reach the nominal energy of 14 $TeV$ at the LHC. The 2011 data analysis has been conducted in parallel with the study of the following signal processes: $H^0Z^0\rightarrow\mu^+\mu^-$, $H^0W^\pm\rightarrow\mu^\pm\nu$ both in associated production, and the following background processes: $Z^0Z^0$, $Z^0W^\pm$, $Z^0+b \bar b$, $Z^0+c \bar c$, $W^\pm+b \bar b$, $W^\pm+c \bar c$, $t\bar t$ and $t(\bar t)+X$. The $H^0\rightarrow ZZ^*\rightarrow\mu^+\mu^-l^+l\ ^-$ channel has also been investigated. Moreover, we give the most relevant cross-section values for those processes computed for the LHCb acceptance. Jets coming from $b$ quarks hadronizations are reconstructed and tagged as part of the search for Higgs in associated production events. Muons coming from $Z^0$ or $W^\pm$ bosons are selected choosing cuts calibrated on $Z^0\rightarrow\mu^+\mu^-$ real and MC events, allowing an optimal muon selection. The number of associated Higgs events in 2011 (with an integrated luminosity of 1019 $pb^{-1}$) in a mass range from 90 to 140 $GeV/c^2$ is $0.0673\pm0.0055$. The number of background events, from MC11, is evaluated to be $2.94^{+1.33}_{-0.61}$. We did not find any event in this mass range for the year 2011. An upper limit on the production cross-section for the $H^0Z^0$ and $H^0W^\pm$ signal channels has been computed to be $\sigma_{sup}(H^0Z^0/W^\pm)=20.5\, pb\,\, CL=95\%$.