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.
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 top-antitop quark (t t-bar) production cross section is measured in proton-proton collisions at sqrt(s)=8 TeV with the CMS experiment at the LHC, using a data sample corresponding to an ...integrated luminosity of 5.3 inverse femtobarns. 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 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.
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 mass of the top quark is measured using a sample of ttbar candidate events with at least six jets in the final state. The sample is selected from data collected with the CMS detector in pp ...collisions at sqrt(s)=7 TeV in 2011 and corresponds to an integrated luminosity of 3.54 inverse femtobarns. The mass is reconstructed for each event employing a kinematic fit of the jets to a ttbar hypothesis. The top-quark mass is measured to be 173.49 +/- 0.69 (stat.) +/- 1.21 (syst.) GeV. A combination with previously published measurements in other decay modes by CMS yields a mass of 173.54 +/- 0.33 (stat.) +/- 0.96 (syst.) GeV.
Spectra of identified charged hadrons are measured in pPb collisions with the CMS detector at the LHC at sqrt(sNN) =5.02 TeV. Charged pions, kaons, and protons in the transverse-momentum range pt ...approximately 0.1-1.7 GeV and laboratory rapidity abs(y) < 1 are identified via their energy loss in the silicon tracker. The average pt increases with particle mass and the charged multiplicity of the event. The increase of the average pt with charged multiplicity is greater for heavier hadrons. Comparisons to Monte Carlo event generators reveal that EPOS LHC, which incorporates additional hydrodynamic evolution of the created system, is able to reproduce most of the data features, unlike HIJING and AMPT. The pt spectra and integrated yields are also compared to those measured in pp and PbPb collisions at various energies. The average transverse momentum and particle ratio measurements indicate that particle production at LHC energies is strongly correlated with event particle multiplicity.
Results are presented from a search for a narrow, spin-2 resonance decaying into a pair of Z bosons, with one Z-boson decaying into leptons (e+e- or mu+mu-) and the other into jets. An example of ...such a resonance is the Kaluza--Klein graviton, GKK, predicted in Randall--Sundrum models. The analysis is based on a 4.9 inverse femtobarn sample of proton-proton collisions at a center-of-mass energy of 7 TeV, collected with the CMS detector at the LHC. Kinematic and topological properties, including decay angular distributions as a novel feature of the analysis, are used to discriminate between signal and background. No evidence for a resonance is observed, and upper limits on the production cross sections times branching fractions are set. In two models that predict Z-boson spin correlations in graviton decays, graviton masses are excluded lower than a value which varies between 610 and 945 GeV, depending on the model and the strength of the graviton couplings.
Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million ...collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, eta, and full azimuth, phi, as a function of charged particle multiplicity and particle transverse momentum, pt. In high-multiplicity events, a long-range (2
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