The top-quark pair-production cross-section measurement in the dilepton final states at proton-proton collisions with $\sqrt{s}=$7~TeV

This thesis presents the results of the top-quark pair-production ($t\bar{t}$ production) cross-section measurement at proton-proton collisions with $\sqrt{s} = 7~{\rm TeV}$ at the Large Hadron Collider (LHC), which is the energy frontier accelerator as of 2011. The top-quark ($t$-quark) is the heaviest of all known particles, and it was discovered at the Tevatron with $\sqrt{s} = 1.96~{\rm TeV}$ in 1995. Owing to its largest mass, the $t$-quark is expected to be sensitive for new phenomena, and thus a precise measurement of the $t\bar{t}$ production at the energy frontier is one of the first priorities of experimental particle physics programs. This analysis uses $0.70~{fb}^{-1}$ of $pp$ collision data corresponding to production of $10^{5}$ times of the $t\bar{t}$ creations, which has already exceeded the number of $t\bar{t}$ pairs produced at the Tevatron. The analysis extracts the $t\bar{t}$ events in final states characterized by two isolated high $p_{T}$ leptons, a large missing transverse momentum ($E_{T}^{miss}$) corresponding to two neutrinos from leptonic $W$ boson's decays, and jets originating from b-quarks, which are so-called ``dilepton final states''. The measurement is based on the well-evaluated detector performances especially in the high intensity environment of the LHC. The following has been achieved owing to the intensive study of the detector performance: (1) the optimization of the $t\bar{t}$ event selection, (2) the evaluation of the signal acceptance, and (3) the background rate estimation. Thus far, 1,400 candidate events were obtained from the $pp$ collision data with expected the signal purity of 83\%. Using the obtained samples, the cross-section is measured to be: $175 \pm6 {\rm (stat.)}\ ^{+14}_{-11} {\rm (syst.)}\ \pm8 {\rm (lumi.)}\ {\rm pb},$ where stat, syst, and lumi denote the statistical error, the sum of systematic errors, and the uncertainty originating from the luminosity determination, respectively. This result is in good agreement with the SM prediction of $164.6\ ^{+11.5}_{-15.8}$~pb. The uncertainty level achieved by this analysis is less than 10\% and compatible to that of the SM theory. The research covers following achievements: (1) Confirmation of the presence of $t$-quark at the LHC (2) Establishment of the method to identify $t\bar{t}$ events at the LHC $pp$ collisions and to evaluate the background contamination. (3) The first precise measurement of the $t\bar{t}$ production rates in the dilepton final state at the 7~TeV collisions (4) First detailed study of the $t\bar{t}$ production kinematic properties in the LHC which confirms the current particle physics Standard Model describes them well. The methods developed and the studies performed for the $t\bar{t}$ cross-section measurement are applicable for other physics analyses using $t$-quark, e.g. a new physics search program with signatures of $t\bar{t}+E_{T}^{miss}$ or $t\bar{t}+$jets. The $t\bar{t}$ identification method established in the present work forms the basis for these physics programs. For analyses in which the $t\bar{t}$ production is a dominating background such as the supersymmetry search program, the $t\bar{t}$ background can be evaluated with reference to this experimental result, and it would improve their sensitivity.