The normalised differential top quark-antiquark production cross section is measured as a function of the jet multiplicity in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC ...with the CMS detector. The measurement is performed in both the dilepton and lepton+jets decay channels using data corresponding to an integrated luminosity of 5.0 inverse femtobarns. Using a procedure to associate jets to decay products of the top quarks, the differential cross section of the t t-bar production is determined as a function of the additional jet multiplicity in the lepton+jets channel. Furthermore, the fraction of events with no additional jets is measured in the dilepton channel, as a function of the threshold on the jet transverse momentum. The measurements are compared with predictions from perturbative quantum chromodynamics and no significant deviations are observed.
A search for excited quarks decaying into the photon+jet final state is presented. The analysis is based on data corresponding to an integrated luminosity of 19.7 inverse femtobarns collected by the ...CMS experiment in proton-proton collisions at sqrt(s)=8 TeV at the LHC. Events with photons and jets with high transverse momenta are selected and the photon+jet invariant mass distribution is studied to search for a resonance peak. The 95% confidence level upper limits on the product of cross section and branching fraction are evaluated as a function of the excited quark mass. Limits on excited quarks are presented as a function of their mass and coupling strength; masses below 3.5 TeV are excluded at 95% confidence level for unit couplings to their standard model partners.
The top-quark pair production cross section is measured in final states with one electron or muon and one hadronically decaying tau lepton from the process ttbar to (l nul) (tau nutau) bbbar, where l ...= e, mu. The data sample corresponds to an integrated luminosity of 19.6 inverse femtobarns collected with the CMS detector in proton-proton collisions at sqrt(s)=8 TeV. The measured cross section sigmattbar = 257 +/- 3 (stat) +/- 24 (syst) +/- 7 (lum) pb, assuming a top-quark mass of 172.5 GeV, is consistent with the standard model prediction.
The purely electroweak (EW) cross section for the production of two jets in association with a Z boson, in proton-proton collisions at sqrt(s) = 8 TeV, is measured using data recorded by the CMS ...experiment at the CERN LHC, corresponding to an integrated luminosity of 19.7 inverse femtobarns. The electroweak cross section for the lljj final state (with l = e or mu and j representing the quarks produced in the hard interaction) in the kinematic region defined by Mll > 50 GeV, Mjj > 120 GeV, transverse momentum ptj > 25 GeV, and pseudorapidity abs(etaj) < 5, is found to be sigmaEW(lljj) = 174 +/- 15 (stat) +/- 40 (syst) fb, in agreement with the standard model prediction. The associated jet activity of the selected events is studied, in particular in a signal-enriched region of phase space, and the measurements are found to be in agreement with QCD predictions.
Transverse momentum spectra of charged particles are measured by the CMS experiment at the CERN LHC in pPb collisions at sqrt(sNN) =5.02 TeV, in the range 0.4 < pt < 120 GeVc and pseudorapidity ...abs(etaCM) < 1.8 in the proton-nucleon center-of-mass frame. For pt < 10 GeV, the charged-particle production is asymmetric about etaCM = 0, with smaller yield observed in the direction of the proton beam, qualitatively consistent with expectations from shadowing in nuclear parton distribution functions (nPDF). A pp reference spectrum at sqrt(s) = 5.02 TeV is obtained by interpolation from previous measurements at higher and lower center-of-mass energies. The pt distribution measured in pPb collisions shows an enhancement of charged particles with pt > 20 GeV compared to expectations from the pp reference. The enhancement is larger than predicted by perturbative quantum chromodynamic calculations that include antishadowing modifications of nPDFs.
Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at sqrt(sNN) = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set ...includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 inverse microbarns. The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v2 to v6, are extracted as a function of particle transverse momentum. At higher transverse momentum, the v2 harmonic becomes significantly smaller than the higher-order vn (n greater than or equal to 3). The pt-averaged v2 and v3 are found to be equal within 2%, while higher-order vn decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-by-event fluctuating initial conditions is also presented.
Measurements are presented of the t-channel single-top-quark production cross section in proton-proton collisions at sqrt(s) = 8 TeV. The results are based on a data sample corresponding to an ...integrated luminosity of 19.7 inverse femtobarns recorded with the CMS detector at the LHC. The cross section is measured inclusively, as well as separately for top (t) and antitop (t-bar), in final states with a muon or an electron. The measured inclusive t-channel cross section is sigmat-ch = 83.6 +/- 2.3 (stat.) +/- 7.4 (syst.) pb. The single t and t-bar cross sections are measured to be sigmat-ch,t = 53.8 +/- 1.5 (stat.) +/- 4.4 (syst.) pb and sigmat-ch,t-bar = 27.6 +/- 1.3 (stat.)+/- 3.7 (syst.) pb, respectively. The measured ratio of cross sections is Rt-ch=sigmat-ch,t/sigmat-ch,t-bar=1.95 +/- 0.10 (stat.) +/- 0.19 (syst.), in agreement with the standard model prediction. The modulus of the Cabibbo-Kobayashi-Maskawa matrix element Vtb is extracted and, in combination with a previous CMS result at sqrt(s) = 7 TeV, a value abs(Vtb) = 0.998 +/- 0.038 (exp.) +/- 0.016 (theo.) is obtained.
The t t-bar charge asymmetry in proton-proton collisions at sqrt(s)=7 TeV is measured using the dilepton decay channel (ee, e mu, or mu mu). The data correspond to a total integrated luminosity of ...5.0 inverse femtobarns, collected by the CMS experiment at the LHC. The t t-bar and lepton charge asymmetries, defined as the differences in absolute values of the rapidities between the reconstructed top quarks and antiquarks and of the pseudorapidities between the positive and negative leptons, respectively, are measured to be Ac = -0.010 +/- 0.017 (stat.) +/- 0.008 (syst.) and Aclep = 0.009 +/- 0.010 (stat.) +/- 0.006 (syst). The lepton charge asymmetry is also measured as a function of the invariant mass, rapidity, and transverse momentum of the t t-bar system. All measurements are consistent with the expectations of the standard model.
The ratio between the prompt psi(2S) and J/psi yields, reconstructed via their decays into muon pairs, is measured in PbPb and pp collisions at sqrt(sNN) = 2.76 TeV. The analysis is based on PbPb and ...pp data samples collected by CMS at the LHC, corresponding to integrated luminosities of 150 inverse microbarns and 5.4 inverse picobarns, respectively. The double ratio of measured yields, (Npsi(2S)/NJ/psi)PbPb / (Npsi(2S)/ NJ/psi)pp, is computed in three PbPb collision centrality bins and two kinematic ranges: one at midrapidity, abs(y)<1.6, covering the transverse momentum range 6.5
The inclusive cross section for top-quark pair production measured by the CMS experiment in proton-proton collisions at a center-of-mass energy of 7 TeV is compared to the QCD prediction at ...next-to-next-to-leading order with various parton distribution functions to determine the top-quark pole mass, mtpole, or the strong coupling constant, alphaS. With the parton distribution function set NNPDF2.3, a pole mass of 176.7+3.8-3.4 GeV is obtained when constraining alphaS at the scale of the Z boson mass, mZ, to the current world average. Alternatively, by constraining mtpole to the latest average from direct mass measurements, a value of alphaS(mZ) = 0.1151+0.0033-0.0032 is extracted. This is the first determination of alphaS using events from top-quark production.
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