Browsing by Author "Boulahouache, C."
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Item Azimuthal Anisotropy of Charged Particles at High Transverse Momenta in Pb-Pb Collisions at √sNN=2.76 TeV(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Cuplov, V.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The azimuthal anisotropy of charged particles in Pb-Pb collisions at √sNN=2.76 TeV is measured with the CMS detector at the LHC over an extended transverse momentum (pT) range up to approximately 60 GeV/c. The data cover both the low-pT region associated with hydrodynamic flow phenomena and the high-pT region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter (v2) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0–60% most-central events, the observed v2 values are found to first increase with pT, reaching a maximum around pT=3 GeV/c, and then to gradually decrease to almost zero, with the decline persisting up to at least pT=40 GeV/c over the full centrality range measured.Item Combined results of searches for the standard model Higgs boson in pp collisions at √s = 7 TeV(Elsevier, 2012) Adair, A.; Boulahouache, C.; Cuplov, V.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Combined results are reported from searches for the standard model Higgs boson in proton–proton collisions at √s=7 TeV in five Higgs boson decay modes: γγ, bb, ττ, WW, and ZZ. The explored Higgs boson mass range is 110–600 GeV. The analysed data correspond to an integrated luminosity of 4.6–4.8 fb−1. The expected excluded mass range in the absence of the standard model Higgs boson is 118–543 GeV at 95% CL. The observed results exclude the standard model Higgs boson in the mass range 127-600 GeV at 95% CL, and in the mass range 129–525 GeV at 99% CL. An excess of events above the expected standard model background is observed at the low end of the explored mass range making the observed limits weaker than expected in the absence of a signal. The largest excess, with a local significance of 3.1σ, is observed for a Higgs boson mass hypothesis of 124 GeV. The global significance of observing an excess with a local significance ⩾3.1σ anywhere in the search range 110–600 (110–145) GeV is estimated to be 1.5σ(2.1σ). More data are required to ascertain the origin of the observed excessItem Combined search for the quarks of a sequential fourth generation(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Results are presented from a search for a fourth generation of quarks produced singly or in pairs in a data set corresponding to an integrated luminosity of 5 fb−1 recorded by the CMS experiment at the LHC in 2011. A novel strategy has been developed for a combined search for quarks of the up and down type in decay channels with at least one isolated muon or electron. Limits on the mass of the fourth-generation quarks and the relevant Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a simple extension of the standard model with a sequential fourth generation of fermions. The existence of mass-degenerate fourth-generation quarks with masses below 685 GeV is excluded at 95% confidence level for minimal off-diagonal mixing between the third- and the fourth-generation quarks. With a mass difference of 25 GeV between the quark masses, the obtained limit on the masses of the fourth-generation quarks shifts by about ±20 GeV. These results significantly reduce the allowed parameter space for a fourth generation of fermionsItem Erratum: Search for anomalous t¯t production in the highly-boosted all-hadronic final state(Springer, 2014) Adair, A.; Boulahouache, C.; Cuplov, V.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Item Event shapes and azimuthal correlations in Z+jets events in pp collisions at √s=7 TeV(Elsevier, 2013) Adair, A.; Akgun, B.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton–proton collisions. The data collected with the CMS detector at the CERN LHC at √s=7 TeV correspond to an integrated luminosity of 5.0 fb−1. The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z+1-jet events. The experimental results are corrected for detector effects, and can be compared directly with other QCD models.Item Evidence for Associated Production of a Single Top Quark and W Boson in pp Collisions at √s=7 TeV(American Physical Society, 2013) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Evidence is presented for the associated production of a single top quark and W boson in pp collisions at √s=7 TeV with the CMS experiment at the LHC. The analyzed data correspond to an integrated luminosity of 4.9 fb−1. The measurement is performed using events with two leptons and a jet originated from a b quark. A multivariate analysis based on kinematic properties is utilized to separate the tt¯ background from the signal. The observed signal has a significance of 4.0σ and corresponds to a cross section of 16+5−4 pb, in agreement with the standard model expectation of 15.6±0.4+1.0−1.2 pb.Item Interpretation of searches for supersymmetry with simplified models(American Physical Society, 2013) Adair, A.; Akgun, B.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The results of searches for supersymmetry by the CMS experiment are interpreted in the framework of simplified models. The results are based on data corresponding to an integrated luminosity of 4.73 to 4.98 fb−1. The data were collected at the LHC in proton–proton collisions at a center-of-mass energy of 7 TeV. This paper describes the method of interpretation and provides upper limits on the product of the production cross section and branching fraction as a function of new particle masses for a number of simplified models. These limits and the corresponding experimental acceptance calculations can be used to constrain other theoretical models and to compare different supersymmetry-inspired analyses.Item Measurement of associated production of vector bosons and top quark-antiquark pairs in pp collisions at s =7 TeV(American Physical Society, 2013-04) Adair, A.; Akgun, B.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Padley, B.P.Item Measurement of the Azimuthal Anisotropy of Neutral Pions in Pb-Pb Collisions at √sNN =2.76 TeV(American Physical Society, 2013) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.First measurements of the azimuthal anisotropy of neutral pions produced in Pb-Pb collisions at a center-of-mass energy of √sNN=2.76 TeV are presented. The amplitudes of the second Fourier component (v2) of the π0 azimuthal distributions are extracted using an event-plane technique. The values of v2 are studied as a function of the neutral pion transverse momentum (pT) for different classes of collision centrality in the kinematic range 1.6Item Measurement of the Electron Charge Asymmetry in Inclusive W Production in pp Collisions at √s=7 TeV(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A measurement of the electron charge asymmetry in inclusive pp→W+X→eν+X production at √s=7 TeV is presented based on data recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 840 pb−1. The electron charge asymmetry reflects the unequal production of W+ and W− bosons in pp collisions. The electron charge asymmetry is measured in bins of the absolute value of electron pseudorapidity in the range of |η|<2.4. The asymmetry rises from about 0.1 to 0.2 as a function of the pseudorapidity and is measured with a relative precision better than 7%. This measurement provides new stringent constraints for parton distribution functions.Item Measurement of the Pseudorapidity and Centrality Dependence of the Transverse Energy Density in Pb-Pb Collisions at √sNN=2.76 TeV(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The transverse energy (ET) in Pb-Pb collisions at 2.76 TeV nucleon-nucleon center-of-mass energy (√sNN) has been measured over a broad range of pseudorapidity (η) and collision centrality by using the CMS detector at the LHC. The transverse energy density per unit pseudorapidity (dET/dη) increases faster with collision energy than the charged particle multiplicity. This implies that the mean energy per particle is increasing with collision energy. At all pseudorapidities, the transverse energy per participating nucleon increases with the centrality of the collision. The ratio of transverse energy per unit pseudorapidity in peripheral to central collisions varies significantly as the pseudorapidity increases from η=0 to |η|=5.0. For the 5% most central collisions, the energy density per unit volume is estimated to be about 14 GeV/fm3 at a time of 1 fm/c after the collision. This is about 100 times larger than normal nuclear matter density and a factor of 2.6 times higher than the energy density reported at √sNN=200 GeV at the Relativistic Heavy Ion Collider.Item Measurements of differential jet cross sections in proton-proton collisions at √s=7 TeV with the CMS detector(American Physical Society, 2013) Adair, A.; Akgun, B.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Li, W.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Measurements of inclusive jet and dijet production cross sections are presented. Data from LHC proton-proton collisions at √s=7 TeV, corresponding to 5.0 fb−1 of integrated luminosity, have been collected with the CMS detector. Jets are reconstructed up to rapidity 2.5, transverse momentum 2 TeV, and dijet invariant mass 5 TeV, using the anti-kT clustering algorithm with distance parameter R=0.7. The measured cross sections are corrected for detector effects and compared to perturbative QCD predictions at next-to-leading order, using five sets of parton distribution functions.Item Observation of a diffractive contribution to dijet production in proton-proton collisions at √s=7 TeV(American Physical Society, 2013) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The cross section for dijet production in proton-proton collisions at √s=7 TeV is presented as a function of ξ˜, a variable that approximates the fractional momentum loss of the scattered proton in single-diffractive events. The analysis is based on an integrated luminosity of 2.7 nb−1 collected with the CMS detector at the LHC at low instantaneous luminosities, and uses events with jet transverse momentum of at least 20 GeV. The dijet cross section results are compared to the predictions of diffractive and nondiffractive models. The low-ξ˜ data show a significant contribution from diffractive dijet production, observed for the first time at the LHC. The associated rapidity gap survival probability is estimated.Item Observation of a New Ξb Baryon(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The observation of a new b baryon via its strong decay into Ξ−bπ+ (plus charge conjugates) is reported. The measurement uses a data sample of pp collisions at √s=7 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 5.3 fb−1. The known Ξ−b baryon is reconstructed via the decay chain Ξ−b→J/ψΞ−→μ+μ−Λ0π−, with Λ0→pπ−. A peak is observed in the distribution of the difference between the mass of the Ξ−bπ+ system and the sum of the masses of the Ξ−b and π+, with a significance exceeding 5 standard deviations. The mass difference of the peak is 14.84±0.74(stat)±0.28(syst) MeV. The new state most likely corresponds to the JP=3/2+ companion of the Ξb.Item Observation of Sequential Υ Suppression in PbPb Collisions(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The suppression of the individual Υ(nS) states in PbPb collisions with respect to their yields in pp data has been measured. The PbPb and pp data sets used in the analysis correspond to integrated luminosities of 150 μb−1 and 230 nb−1, respectively, collected in 2011 by the CMS experiment at the LHC, at a center-of-mass energy per nucleon pair of 2.76 TeV. The Υ(nS) yields are measured from the dimuon invariant mass spectra. The suppression of the Υ(nS) yields in PbPb relative to the yields in pp, scaled by the number of nucleon-nucleon collisions, RAA, is measured as a function of the collision centrality. Integrated over centrality, the RAA values are 0.56±0.08(stat)±0.07(syst), 0.12±0.04(stat)±0.02(syst), and lower than 0.10 (at 95% confidence level), for the Υ(1S), Υ(2S), and Υ(3S) states, respectively. The results demonstrate the sequential suppression of the Υ(nS) states in PbPb collisions at LHC energies.Item Search for a Light Pseudoscalar Higgs Boson in the Dimuon Decay Channel in pp Collisions at √s=7 TeV(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.The dimuon invariant mass spectrum is searched in the range between 5.5 and 14 GeV for a light pseudoscalar Higgs boson a, predicted in a number of new physics models, including the next-to-minimal supersymmetric standard model. The data sample used in the search corresponds to an integrated luminosity of 1.3 fb−1 collected in pp collisions at √s=7 TeV with the CMS detector at the LHC. No excess is observed above the background predictions and upper limits are set on the cross section times branching fraction σ×B(pp→a→μ+μ−) in the range of 1.5–7.5 pb. These results improve on existing bounds on the abb¯ coupling for mamΥ(3S). Constraints on the supersymmetric parameter space are presented in the context of the next-to-minimal model.l.Item Search for a Vectorlike Quark with Charge 2/3 in t+Z Events from pp Collisions at √s=7 TeV(American Physical Society, 2011) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A search for pair-produced heavy vectorlike charge-2/3 quarks, T, in pp collisions at a center-of-mass energy of 7 TeV, is performed with the CMS detector at the LHC. Events consistent with the flavor-changing-neutral-current decay of a T quark to a top quark and a Z boson are selected by requiring two leptons from the Z-boson decay, as well as an additional isolated charged lepton. In a data sample corresponding to an integrated luminosity of 1.14 fb−1, the number of observed events is found to be consistent with the standard model background prediction. Assuming a branching fraction of 100% for the decay T→tZ, a T quark with a mass less than 475 GeV/c2 is excluded at the 95% confidence level.Item Search for a W′ or Techni-ρ Decaying into WZ in pp Collisions at √s=7 TeV(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.A search is performed in pp collisions at √s=7 TeV for exotic particles decaying via WZ to final states with electrons and muons. The data sample corresponds to an integrated luminosity of approximately 5 fb−1. No significant excess is observed in the data above the expected standard model background. Upper bounds at 95% confidence level are set on the production cross section of the W′ boson described by the sequential standard model and on the W′ WZ coupling. W′ bosons with masses below 1143 GeV are excluded. Limits are also set in the context of low-scale technicolor models, under a range of assumptions concerning the model parameters.Item Search for contact interactions in μ+μ− events in pp collisions at √s=7 TeV(American Physical Society, 2013) Adair, A.; Boulahouache, C.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Results are reported from a search for the effects of contact interactions using events with a high-mass, oppositely charged muon pair. The events are collected in proton-proton collisions at √s=7 TeV using the Compact Muon Solenoid detector at the Large Hadron Collider. The data sample corresponds to an integrated luminosity of 5.3 fb−1. The observed dimuon mass spectrum is consistent with that expected from the standard model. The data are interpreted in the context of a quark- and muon-compositeness model with a left-handed isoscalar current and an energy scale parameter Λ. The 95% confidence level lower limit on Λ is 9.5 TeV under the assumption of destructive interference between the standard model and contact-interaction amplitudes. For constructive interference, the limit is 13.1 TeV. These limits are comparable to the most stringent ones reported to date.Item Search for Dark Matter and Large Extra Dimensions in pp Collisions Yielding a Photon and Missing Transverse Energy(American Physical Society, 2012) Adair, A.; Boulahouache, C.; Cuplov, V.; Ecklund, K.M.; Geurts, F.J.M.; Padley, B.P.; Redjimi, R.; Roberts, J.; Zabel, J.Results are presented from a search for new physics in the final state containing a photon (γ) and missing transverse energy (E̸T). The data correspond to an integrated luminosity of 5.0 fb−1 collected in pp collisions at √s=7 TeV by the CMS experiment. The observed event yield agrees with standard-model expectations for the γ+E̸T events. Using models for the production of dark-matter particles (χ), we set 90% confidence level (C.L.) upper limits of 13.6–15.4 fb on χ production in the γ+E̸T state. These provide the most sensitive upper limits for spin-dependent χ-nucleon scattering for χ masses (Mχ) between 1 and 100 GeV. For spin-independent contributions, the present limits are extended to Mχ<3.5 GeV. For models with 3–6 large extra dimensions, our data exclude extra-dimensional Planck scales between 1.64 and 1.73 TeV at 95% C.L.