11 research outputs found

    Observation of top quark pairs produced in association with a vector boson in pp collisions at s=8 √s=8TeV

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    Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb −1 , collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from t t ¯ W tt¯W or t t ¯ Z tt¯Z decays. The t t ¯ W tt¯W cross section is measured to be 382 − 102 + 117 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The t t ¯ Z tt¯Z cross section is measured to be 242 − 55 + 65 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the t t ¯ W tt¯W and t t ¯ Z tt¯Z cross sections

    Erratum: Measurement of the t(t)over-bar production cross section in the dilepton channel in pp collisions at root s = 8 TeV (vol 2, 024, 2014)

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    Event generator tunes obtained from underlying event and multiparton scattering measurements

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    New sets of parameters (“tunes”) for the underlying-event (UE) modelling of the pythia8, pythia6 and herwig++ Monte Carlo event generators are constructed using different parton distribution functions. Combined fits to CMS UE proton–proton (pp) data at √s =7TeV and to UE proton–antiproton (pp ¯) data from the CDF experiment at lower √s, are used to study the UE models and constrain their parameters, providing thereby improved predictions for proton–proton collisions at 13TeV. In addition, it is investigated whether the values of the parameters obtained from fits to UE observables are consistent with the values determined from fitting observables sensitive to double-parton scattering processes. Finally, comparisons are presented of the UE tunes to “minimum bias” (MB) events, multijet, and Drell–Yan (qq ¯ →Z/γ ∗ → lepton-antilepton+jets) observables at 7 and 8TeV, as well as predictions for MB and UE observables at 13TeV.

    Search for a massive resonance decaying into a Higgs boson and a W or Z boson in hadronic final states in proton-proton collisions at root s=8 TeV

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    Search for direct pair production of scalar top quarks in the single- and dilepton channels in proton-proton collisions at root s=8 TeV

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    Strange hadron collectivity in pPb and PbPb collisions

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    Abstract The collective behavior of K S 0 KS0 {\textrm{K}}_{\textrm{S}}^0 and Λ / Λ ¯ Λ/Λ \Lambda /\overline{\Lambda} strange hadrons is studied by measuring the elliptic azimuthal anisotropy (v 2) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy s NN sNN \sqrt{s_{\textrm{NN}}} = 8.16 TeV and lead-lead (PbPb) collisions at s NN sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20 GeV is present. The strange hadron v 2 values extracted in pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size

    Strange hadron collectivity in pPb and PbPb collisions

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    The collective behavior of KS0 {\textrm{K}}_{\textrm{S}}^0 and Λ/Λ \Lambda /\overline{\Lambda} strange hadrons is studied by measuring the elliptic azimuthal anisotropy (v2_{2}) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy sNN \sqrt{s_{\textrm{NN}}} = 8.16 TeV and lead-lead (PbPb) collisions at sNN \sqrt{s_{\textrm{NN}}} = 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20 GeV is present. The strange hadron v2_{2} values extracted in pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size.[graphic not available: see fulltext

    Strange hadron collectivity in pPb and PbPb collisions

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    Measurement of dijet azimuthal decorrelation in pp collisions at √s=8TeV

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    A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2TeV. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8TeV corresponding to an integrated luminosity of 19.7fb-1. The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production. © 2016, CERN for the benefit of the CMS collaboration
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