Physique du quark top dans l'expérience CMS au démarrage du LHC

The first part of this thesis is about the improve made to the seeding algorithm of track reconstruction for the hadrons and the muons in the silicon tracker of the CMS experiment. The different stages from the creation to the tests, which allowed us to qualify this new algorithm as the standard seeding for tracks reconstruction, are presented in this document. The second part is dedicated to the creation of an alternative method to measure the cross-section of the top-antitop pairs production in the CMS experiment at the LHC launch. This analysis has been made using the channel of the semi-muonic decay with at least one another muon coming from a bottom quark and has been studied on full simulation showing the feasibility to “re-discover” the top quark with 5pb−1. The 2.4pb−1 of data collected by the end of august have allowed me to observe the first top-antitop pairs and to make the first cross-section measurement: 171 ± 77(stat.) ± 27(syst.) pb