[eng] This thesis presents the measurement of the top quark pair production cross section in the CMS detector at the CERN Large Hadron Collider (LHC).
At LHC, the top quark is produced predominantly together with its anti-particle via the strong interaction. The top and the anti-top then decay almost instantaneously into three possible final states. We have shown that the dileptonic final state, composed of two opposite sign charged leptons, two neutrinos and two jets is a clear signature with very low contamination from background processes and is therefore ideal for an early observation.
The results have been obtained from data harvested between March and September 2010 in proton-proton collisions at a centre-of-mass energy of 7 TeV, corresponding to an accumulated luminosity of 3.1/pb. Given the complexity of the topology and possible uncertainties related to the quality of early data, special care on the reconstruction techniques are applied that guaranty the robustness of the analysis. Thus, two scenarios have been developed: The first is based on a minimal use of the CMS calorimeters and relies mainly on the CMS tracker subdetector information; The second makes use of the whole detector.
The two scenarios are complementary and allow a direct cross-check, increasing the confidence on the results. Methods to estimate the amount of dominant backgrounds from data (the Drell-Yan process and the contribution from non-prompt or fake leptons) are developed. The cross section, extracted from a simple counting method and taking into account the systematic uncertainties, is: 194 ± 72 (stat.) ± 24 (syst.) ± 21 (lumi.) pb.