2,325 research outputs found

    Studies of azimuthal dihadron correlations in ultra-central PbPb collisions at sNN−−−−√ =2.76 TeV

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    This is the published version.Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at sNN−−−−√ = 2.76TeV 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 μb − 1. 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 v 2 to v 6, are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T-averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n 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

    Measurement of differential cross sections for the production of a pair of isolated photons in pp collisions at sqrt(s) = 7 TeV

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    This is the published version.A measurement of differential cross sections for the production of a pair of isolated photons in proton–proton collisions at s√=7TeV is presented. The data sample corresponds to an integrated luminosity of 5.0fb−1 collected with the CMS detector. A data-driven isolation template method is used to extract the prompt diphoton yield. The measured cross section for two isolated photons, with transverse energy above 40 and 25GeV respectively, in the pseudorapidity range |η|0.45, is 17.2±0.2(stat)±1.9(syst)±0.4(lumi) \,pb. Differential cross sections are measured as a function of the diphoton invariant mass, the diphoton transverse momentum, the azimuthal angle difference between the two photons, and the cosine of the polar angle in the Collins–Soper reference frame of the diphoton system. The results are compared to theoretical predictions at leading, next-to-leading, and next-to-next-to-leading order in quantum chromodynamics

    Search for new physics in events with same-sign dileptons and jets in pp collisions at s√ = 8 TeV

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    This is the published version.A search for new physics is performed based on events with jets and a pair of isolated, same-sign leptons. The results are obtained using a sample of proton-proton collision data collected by the CMS experiment at a centre-of-mass energy of 8 TeV at the LHC, corresponding to an integrated luminosity of 19.5 fb−1. In order to be sensitive to a wide variety of possible signals beyond the standard model, multiple search regions defined by the missing transverse energy, the hadronic energy, the number of jets and b-quark jets, and the transverse momenta of the leptons in the events are considered. No excess above the standard model background expectation is observed and constraints are set on a number of models for new physics, as well as on the same-sign top-quark pair and quadruple-top-quark production cross sections. Information on event selection efficiencies is also provided, so that the results can be used to confront an even broader class of new physics models

    Event activity dependence of Y(nS) production in sNN−−−−√ = 5.02 TeV pPb and s√ = 2.76 TeV pp collisions

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    This is the published version.The production of Y(1S), Y(2S), and Y(3S) is investigated in pPb and pp collisions at centre-of-mass energies per nucleon pair of 5.02 TeV and 2.76 TeV, respectively. The datasets correspond to integrated luminosities of about 31 nb−1 (pPb) and 5.4 pb−1 (pp), collected in 2013 by the CMS experiment at the LHC. Upsilons that decay into muons are reconstructed within the rapidity interval |y CM| < 1.93 in the nucleon-nucleon centre-of-mass frame. Their production is studied as a function of two measures of event activity, namely the charged-particle multiplicity measured in the pseudorapidity interval |η| < 2.4, and the sum of transverse energy deposited at forward pseudorapidity, 4.0 < |η| < 5.2. The Y cross sections normalized by their event activity integrated values, Y(nS)/〈Y(nS)〉, are found to rise with both measures of the event activity in pp and pPb. In both collision systems, the ratios of the excited to the ground state cross sections, Y(nS)/ Y(1S), are found to decrease with the charged-particle multiplicity, while as a function of the transverse energy the variation is less pronounced. The event activity integrated double ratios, [Y(nS)/ Y(1S)]pPb /[Y(nS)/ Y(1S)]pp, are also measured and found to be 0.83 ± 0.05 (stat.) ± 0.05 (syst.) and 0.71 ± 0.08 (stat.) ± 0.09 (syst.) for Y(2S) and Y(3S), respectively

    Search for the associated production of the Higgs boson with a top-quark pair

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    This is the published version.A search for the standard model Higgs boson produced in association with a top-quark pair (tt¯H) is presented, using data samples corresponding to integrated luminosities of up to 5.1 fb−1 and 19.7 fb−1 collected in pp collisions at center-of-mass energies of 7 TeV and 8 TeV respectively. The search is based on the following signatures of the Higgs boson decay: H → hadrons, H → photons, and H → leptons. The results are characterized by an observed tt¯H signal strength relative to the standard model cross section, μ=σ/σ SM,under the assumption that the Higgs boson decays as expected in the standard model. The best fit value is μ = 2.8 ± 1.0 for a Higgs boson mass of 125.6 GeV

    Observation of the diphoton decay of the Higgs boson and measurement of its properties

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    This is the published version.Observation of the diphoton decay mode of the recently discovered Higgs boson and measurement of some of its properties are reported. The analysis uses the entire dataset collected by the CMS experiment in proton-proton collisions during the 2011 and 2012 LHC running periods. The data samples correspond to integrated luminosities of 5.1fb-1at s√=7TeV and 19.7fb-1at 8TeV . A clear signal is observed in the diphoton channel at a mass close to 125GeV with a local significance of 5.7σ, where a significance of 5.2σ is expected for the standard model Higgs boson. The mass is measured to be 124.70±0.34GeV=124.70±0.31(stat)±0.15(syst)GeV , and the best-fit signal strength relative to the standard model prediction is 1.14+0.26−0.23=1.14±0.21(stat) +0.09−0.05(syst) +0.13−0.09(theo). Additional measurements include the signal strength modifiers associated with different production mechanisms, and hypothesis tests between spin-0 and spin-2 models

    Measurement of the production cross sections for a Z boson and one or more b jets in pp collisions at s√ = 7 TeV

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    This is the published version.The production of a Z boson, decaying into two leptons and produced in association with one or more b jets, is studied using proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7 TeV. The data were recorded in 2011 with the CMS detector and correspond to an integrated luminosity of 5 fb−1. The Z(ℓℓ) + b-jets cross sections (where ℓℓ = μμ or ee) are measured separately for a Z boson produced with exactly one b jet and with at least two b jets. In addition, a cross section ratio is extracted for a Z boson produced with at least one b jet, relative to a Z boson produced with at least one jet. The measured cross sections are compared to various theoretical predictions, and the data favour the predictions in the five-flavour scheme, where b quarks are assumed massless. The kinematic properties of the reconstructed particles are compared with the predictions from the MadGraph event generator using the pythia parton shower simulation

    Measurement of the tt¯ production cross section in the dilepton channel in pp collisions at s√ = 8 TeV

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    This is the published version.The top-antitop quark (tt¯) production cross section is measured in proton-proton 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

    Information and Communication Technology Assessment Benchmark

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    This paper reports on an initial effort to assess student learning with respect to effective use of information and communication technology. A business school in a small public university administered the iSkills assessment from the Educational Testing Service (ETS) in 2008. This online web service simulates websites, web searches, databases, and office productivity software to present realistic scenarios. iSkills evaluates seven skill areas (define, access, evaluate, manage, integrate, create, and communicate). Upper division student median performance on the Advanced Level test was generally above the median level of the reference group with strengths in the Define and Integrate proficiencies and weaknesses in the Manage and Create proficiencies. Lower division student median performance on the Advanced Level test was generally below the median with strengths in the Access and the Integrate proficiencies and weaknesses in the Manage, Define, and Communicate proficiencies. Lower division student median performance on the Core Level test was generally below the median with strengths in the Access and Integrate proficiencies and weaknesses in the Evaluate and Communicate proficiencies. While the upper division students performed adequately, there remains room for improvement. The lower division student performance will provide a benchmark to measure the impact of planned curriculum changes
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