1,721,316 research outputs found
Measurement of the ttbar production cross section in the dilepton channel in pp collisions at sqrt(s) =8 TeV
No full textThe top-antitop quark (ttˉ) 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 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
Study of exclusive two-photon production of W(+)W(-) in pp collisions at sqrt(s) = 7 TeV and constraints on anomalous quartic gauge couplings
No full textA search for exclusive or quasi-exclusive W(+)W(-) production by photon-photon interactions, pp to p(*)W(+)W(-)p(*), at sqrt(s) = 7 TeV is reported using data collected by the CMS detector with an integrated luminosity of 5.05 inverse femtobarns. Events are selected by requiring a mu(+/-)e(-/+) vertex with no additional associated charged tracks and dilepton transverse momentum pt(mu(+/-)e(-/+)) > 30 GeV. Two events passing all selection requirements are observed in the data, compared to a standard model expectation of 2.2 +/- 0.4 signal events with 0.84 +/- 0.15 background. The tail of the dilepton pt distribution is studied for deviations from the standard model. No events are observed with pt > 100 GeV. Model-independent upper limits are computed and compared to predictions involving anomalous quartic gauge couplings. The limits on the parameters a[W,(0,C)]/Lambda^2 with a dipole form factor and an energy cutoff Lambda(cutoff) = 500 GeV are of the order of 10E-
Search for supersymmetry in final states with a single lepton, b-quark jets, and missing transverse energy in proton-proton collisions at sqrts=7 TeV
No full textA search motivated by supersymmetric models with light top squarks is presented using proton-proton collision data recorded with the CMS detector at a center-of-mass energy of sqrt(s) = 7 TeV during 2011, corresponding to an integrated luminosity of 4.98 inverse femtobarns. The analysis is based on final states with a single lepton, b-quark jets, and missing transverse energy. Standard model yields are predicted from data using two different approaches. The observed event numbers are found to be compatible with these predictions. Results are interpreted in the context of the constrained minimal supersymmetric standard model and of a simplified model with four top quarks in the final state
Search for supersymmetry with razor variables in pp collisions at sqrt(s) = 7 TeV
No full textThe razor approach to search for R-parity conserving supersymmetric particles is described in detail. Two analyses are considered: an inclusive search for new heavy particle pairs decaying to final states with at least two jets and missing transverse energy, and a dedicated search for final states with at least one jet originating from a bottom quark. For both the inclusive study and the study requiring a bottom-quark jet, the data are examined in exclusive final states corresponding to all-hadronic, single-lepton, and dilepton events. The study is based on the data set of proton-proton collisions at sqrt(s) = 7 TeV collected with the CMS detector at the LHC in 2011, corresponding to an integrated luminosity of 4.7 inverse femtobarns. The study consists of a shape analysis performed in the plane of two kinematic variables, denoted MR and R2, that correspond to the mass and transverse energy flow, respectively, of pair-produced, heavy, new-physics particles. The data are found to be compatible with the background model, defined by studying event simulations and data control samples. Exclusion limits for squark and gluino production are derived in the context of the constrained minimal supersymmetric model (CMSSM) and also for simplified-model spectra (SMS). Within the CMSSM parameter space considered, squark and gluino masses up to 1350 GeV are excluded at 95% confidence level, depending on the model parameters. For SMS scenarios, the direct production of pairs of top or bottom squarks is excluded for masses as high as 400 GeV
Measurement of the properties of a Higgs boson in the four-lepton final state
No full textThe properties of a Higgs boson candidate are measured in the H→ZZ→4l decay channel, with l=e,μ, using data from pp collisions corresponding to an integrated luminosity of 5.1 inverse femtobarns at center-of-mass energy of s√=7 TeV and 19.7 inverse femtobarns at s√=8 TeV, recorded with the CMS detector at the LHC. The new boson is observed as a narrow resonance with a local significance of 6.8 standard deviations, a measured mass of 125.6±0.4 (stat.) ±0.2 (syst.) GeV, and a total width less than 3.4 GeV at a 95% confidence level. The production cross section of the new boson times the branching fraction to four leptons is measured to be 0.93+0.26−0.23(stat.)+0.13−0.09 (syst.) times that predicted by the standard model. Its spin-parity properties are found to be consistent with the expectations for the standard model Higgs boson. The hypotheses of a pseudoscalar and all tested spin-one boson hypotheses are excluded at a 99% confidence level or higher. All tested spin-two boson hypotheses are excluded at a 95% confidence level or higher
Search for Z ' resonances decaying to tbart in dilepton+jets final states in pp collisions at sqrt(s)=7 TeV
No full textA search for resonances decaying to top quark-antiquark pairs is performed using a dilepton+jets data sample recorded by the CMS experiment at the LHC in pp collisions at sqrt(s) = 7 TeV corresponding to an integrated luminosity of 5.0 inverse femtobarns. No significant deviations from the standard model background are observed. Upper limits are presented for the production cross section times branching fraction of top quark-antiquark resonances for masses from 750 to 3000 GeV. In particular, the existence of a leptophobic topcolor particle Z' is excluded at the 95% confidence level for resonance masses M[Z'] < 1.3 TeV for Gamma[Z'] = 0.012 M[Z'], and M[Z'] < 1.9 TeV for Gamma[Z'] = 0.10 M[Z']
Measurement of jet multiplicity distributions in t t-bar production in pp collisions at sqrt(s) = 7 TeV
No full textThe 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
Constraints on the Higgs boson width from off-shell production and decay to Z-boson pairs
No full textConstraints are presented on the total width of the recently discovered Higgs boson, Gamma[H], using its relative on-shell and off-shell production and decay rates to a pair of Z bosons, where one Z boson decays to an electron or muon pair, and the other to an electron, muon, or neutrino pair. The analysis is based on the data collected by the CMS experiment at the LHC in 2011 and 2012, corresponding to integrated luminosities of 5.1 inverse femtobarns at a centre-of-mass energy sqrt(s) = 7 TeV and 19.7 inverse femtobarns at sqrt(s) = 8 TeV. A simultaneous maximum likelihood fit to the measured kinematic distributions near the resonance peak and above the Z-boson pair production threshold leads to an upper limit on the Higgs boson width of Gamma[H] < 22 MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass
Measurement of the underlying event activity in pp collisions at sqrt(s) = 0.9 and 7 TeV with the novel jet-area/median approach
No full textThe first measurement of the charged component of the underlying event using the novel "jet-area/median" approach is presented for proton-proton collisions at centre-of-mass energies of 0.9 and 7 TeV. The data were recorded in 2010 with the CMS experiment at the LHC. A new observable, sensitive to soft particle production, is introduced and investigated inclusively and as a function of the event scale defined by the transverse momentum of the leading jet. Various phenomenological models are compared to data, with and without corrections for detector effects. None of the examined models describe the data satisfactorily
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