Measurement of the tt production cross section and the top quark mass in the dilepton channel in pp collisions at √ s = 7 TeVThe CMS collaboration Abstract: The tt production cross section and top quark mass are measured in protonproton collisions at √ s = 7 TeV in a data sample corresponding to an integrated luminosity of 36 pb −1 collected by the CMS experiment. The measurements are performed in events with two leptons (electrons or muons) in the final state. Results of the cross section measurement in events with and without b-quark identification are obtained and combined. The measured value is σ tt = 168 ± 18 (stat.) ± 14 (syst.) ± 7 (lumi.) pb, consistent with predictions from the standard model. The top quark mass m top is reconstructed with two different methods, a full kinematic analysis and a matrix weighting technique. The combination yields a measurement of m top = 175.5 ± 4.6 (stat.) ± 4.6 (syst.) GeV/c 2 .
Keywords: Hadron-Hadron ScatteringOpen , top quark processes can now be studied extensively in multi-TeV proton-proton (pp) collisions [5,6]. In both pp and pp collisions, top quarks are produced primarily in top-antitop (tt) quark pairs via the strong interaction. At the LHC, the tt production mechanism is dominated by the gluon fusion process, whereas at the Tevatron, top quark pairs are predominantly produced through quark-antiquark annihilation. Measurements of top quark production at the LHC are therefore important new tests of our understanding of the tt production mechanism. The top quark mass is an important parameter of the standard model (SM) and it affects predictions of SM observables via radiative corrections. A precise measurement of the top quark mass is crucial since it constitutes one of the most important inputs to the global electroweak fits [7] that provide constraints on the model itself, including indirect limits on the mass of the Higgs boson. The mass of the top quark has been measured very precisely by the Tevatron experiments, and the current world average is 173.3 ± 0.6 (stat.) ± 0.9 (syst.) GeV/c 2 [8]. Of all quark masses, the mass of the top quark is known with the smallest fractional uncertainty. Within the SM, the top quark decays via the weak process t → Wb almost exclusively. Experimentally, top quark pair events are categorised according to the decay of the two W bosons: the all-hadronic channel, in which both W bosons decay into quarks; the lepton+jets channel, in which one W boson decays leptonically and the other into quarks; and the dilepton channel, in which both W bosons decay into leptons. The measurement described herein is performed using dilepton tt modes (e + e − , µ + µ − , and e ± µ ∓ ). These modes compose (6.45 ± 0.11)% [9] of the total branching fraction for tt when including contributions from tau leptons that subsequently decay to electrons and muons, as is done here. The final state studied in this analysis contains two oppositely charged leptons (electrons or muons), two neutrinos from the W-boson decays, and two jets of particles resulting f...