Measurement of the $ t\overline{t} $ production cross section in the dilepton channel in pp collisions at $ \sqrt{s} $ = 8 TeV

Abstract

The top-antitop quark (tt) production cross section is measured in protonproton collisions at √ s = 8 TeV with the CMS experiment at the LHC, using a data sample corresponding to an integrated luminosity of 5.3 fb −1 . The measurement is performed by analysing events with a pair of electrons or muons, or one electron and one muon, and at least two jets, one of which is identified as originating from hadronisation of a bottom quark. The measured cross section is 239±2 (stat.)±11 (syst.)±6 (lum.) pb, for an assumed top-quark mass of 172.5 GeV, in agreement with the prediction of the standard model. The CMS collaboration 131 IntroductionA precise measurement of the tt production cross section can be used to test the theory of quantum chromodynamics (QCD) at next-to-next-to-leading-order (NNLO) level. It can be also used in global fits of the parton distribution functions (PDF) at NNLO, and allows an estimation of α s (M Z ) as described in [1, 2]. Furthermore, top-quark production is an important source of background in many searches for physics beyond the standard model (SM). A large sample of top-quark events has been collected at the Large Hadron Collider (LHC), and studies of top-quark production have been conducted in various decay channels as well as searches for deviations from the SM predictions [3][4][5][6][7][8][9]. This paper presents a measurement of the tt production cross section, σ tt , based on the dilepton channel (e + e − , µ + µ − , and e ± µ ∓ ) in a data sample of proton-proton collisions at √ s = 8 TeV corresponding to an integrated luminosity of 5.3 fb −1 recorded by the Compact Muon Solenoid (CMS) experiment. In the SM, top quarks are predominantly produced in tt pairs via the strong interaction and decay almost exclusively to a W boson and a bottom quark. We measure the tt production cross section selecting final states that contain two leptons of opposite electric charge, momentum imbalance associated to the neutrinos from the W boson decays, and two jets of particles resulting from the hadronisation of two b quarks. The CMS detector and simulationThe CMS detector [10] has a superconducting solenoid occupying the central region that provides an axial magnetic field of 3.8 T. The silicon pixel and the strip tracker cover 0 < -1 - JHEP02(2014)024φ < 2π in azimuth and |η| < 2.5 in pseudorapidity, where η is defined as η = − ln[tan(θ/2)], with θ being the polar angle measured with respect to the anticlockwise-beam direction. The lead-tungstate crystal electromagnetic calorimeter and the brass/scintillator hadron calorimeter are located inside the solenoid. Muons are measured in gas-ionisation detectors embedded in the steel flux return yoke outside the solenoid. The detector is nearly hermetic, thereby providing reliable measurement of momentum imbalance in the plane transverse to the beams. A two-tier trigger system selects the most interesting pp collisions for offline analysis.Several MC event generators are used to simulate signal and background events: MadGraph (v. 5.1.4.8) [11]...

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