Measurement of the top-antitop and $Z^{0}$-boson production cross sections and their ratio in the dileptonic decay channels at $\sqrt{s} =$ 7 TeV with the CMS experiment

In this thesis, the production of Z0 boson and top-quark pairs is studied. In particular, the decays into two leptons are analyzed. The data recorded by the CMS experiment at the LHC in 2011 is used, corresponding to an integrated luminosity of 1.14 fb−1. The candidates are selected by requiring two isolated opposite-sign lepton candidates. The t¯t and Z0 signal are disentangled by separating the events into two different regions of the invariant mass of the lepton candidate pair. The selection of the t¯t event candidates also includes the requirement for two jets, missing transverse momentum and at least one jet originating from a b-quark. Data-driven estimates for trigger and isolation efficiencies are determined and the electron energies are re-calibrated using the Z0 invariant mass peak. The main backgrounds to the t¯t and to the Z0 events are corrected to or obtained from data, respectively. A Z0 boson production cross section of σZ = 31.1 ±0.9 1.3 (syst.) ±1.4 (lumi.) nb is determined, corresponding to the combination of decay channels into different lepton types. A t¯t production cross section using the dileptonic channels of σt¯t,comb = 181.6 ±3.5 (stat.) ±12.0 12.7 (syst.) ±8.2 (lumi.) pb is obtained, corresponding to the combination of the e+e−, μ+μ− and e±μ∓ channels. A cross section ratio of t¯t to Z0 production of σt¯t/σZ = 5.52 ±0.18 (stat.) ±0.48 0.54 (syst.) ·10−3 is measured. This cross section ratio can be used for an alternative determination of the top-pair production cross section, making use of the precise prediction of the Z0 boson production in the kinematic range of the analysis. The advantage of this method is a cancellation of the luminosity uncertainty. In this way, a value of the top-pair production cross section of σR t¯t,comb = 164.0 ±5.4 (stat.) ±14.2 15.9 (syst.) ±7.1 (theo.) pb is obtained, where (theo.) denotes the uncertainty due to the prediction of the Z0 boson production cross section.