4,118 research outputs found
Charged particle multiplicities in pp interactions at âs = 0.9, 2.36, and 7 TeV
Measurements of primary charged hadron multiplicity distributions are presented for non-single-diractive events in proton-proton collisions at centre-of-mass energies
of âs = 0.9, 2.36, and 7 TeV, in five pseudorapidity ranges from |η| < 0.5 to |η| < 2.4. The data were collected with the minimum-bias trigger of the CMS experiment during the LHC commissioning runs in 2009 and the 7 TeV run in 2010. The multiplicity distribution at âs = 0.9 TeV is in agreement with previous measurements. At higher energies the increase of the mean multiplicity with âs is underestimated by most event generators. The average transverse momentum as a function of the multiplicity is also presented. The measurement of higher-order moments of the multiplicity distribution confirms the violation of
Koba-Nielsen-Olesen scaling that has been observed at lower energies
Observation and studies of jet quenching in PbPb collisions at âsNN = 2.76 TeV
Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the Compact Muon Solenoid (CMS) detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7 ÎŒb^(â1). Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse
momentum is observed, consistent with jet quenching. The observed effect extends from the lower cutoff used
in this study (jet pT = 120 GeV/c) up to the statistical limit of the available data sample (jet pT â 210 GeV/c).
Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by a softening of the fragmentation pattern of the second most energetic, away-side jet. The dijet momentum balance is recovered when integrating low transverse momentum particles distributed over a wide angular range relative to the direction of the away-side jet
Measurements of inclusive W and Z cross sections in pp collisions at âs=7 TeV
Measurements of inclusive W and Z boson production cross sections in pp collisions at âs = 7 TeV are presented, based on 2.9 pb^(-1) of data recorded by the CMS detector at the LHC. The measurements, performed in the electron and muon decay channels, are combined to give Ï(pp â WX) x B (W â lv) = 9.95 ± 0.07 (stat.) ± 0.28 (syst.) ± 1.09 (lumi.) nb and Ï(pp â ZX) x B (Z â â+â-) = 0.931 ± 0.026 (stat.) ± 0.023 (syst.) ± 0.102 (lumi.) nb, where â stands for either e or ”. Theoretical predictions, calculated at the next-to-next-to-leading order in QCD using recent parton distribution functions, are in agreement with the measured cross sections. Ratios of cross sections, which incur an experimental systematic uncertainty of less than 4%, are also reported
Search for resonances in the dilepton mass distribution in pp collisions at âs = 7 TeV
A search for narrow resonances at high mass in the dimuon and dielectron channels has been performed by the CMS experiment at the CERN LHC, using pp collision data recorded at âs=7 TeV. The event samples correspond to integrated luminosities of 40 pb^(â1) in the dimuon channel and 35 pb^(â1) in the dielectron channel. Heavy dilepton resonances are predicted in theoretical models with extra gauge bosons (ZâČ) or as Kaluza-Klein graviton excitations (G_(KK)) in the Randall-Sundrum model. Upper limits on the inclusive cross section of ZâČ(G_(KK)) â â^+ â^â relative to Z â â^+ â^â are presented. These limits exclude at 95% confidence level a ZâČ with standard-model-like couplings below 1140GeV, the superstring-inspired Z'_Ï below 887 GeV, and, for values of the coupling parameter kM_(Pl) of 0.05 (0.1), Kaluza-Klein gravitons below 855 (1079) GeV
Measurement of W^+W^â production and search for the Higgs boson in pp collisions at âs = 7 TeV
A measurement of W^+W^â production in pp collisions at âs = 7 TeV and a search for the Higgs boson are reported. The W^+W^â candidates are selected in events with two leptons, either electrons or muons. The measurement is performed using LHC data recorded with the CMS detector, corresponding to an integrated luminosity of 36 pb^(â1). The pp â W^+W^â cross section is measured to be 41.1±15.3 (stat)±5.8(syst)±4.5(lumi)pb consistent with the standard model prediction. Limits on WWÎł and WWZ anomalous triple gauge couplings are set. The search for the standard model Higgs boson in the W^+W^â decay mode does not reveal any evidence of excess above backgrounds. Limits are set on the production of the Higgs boson in the context of the standard model and in the presence of a sequential fourth family of fermions with high masses. In the latter context, a Higgs boson with mass between 144 and 207 GeV/c^2 is ruled out at 95% confidence level
First measurement of hadronic event shapes in pp collisions at âs = 7 TeV
Hadronic event shapes have been measured in protonâproton collisions at âs = 7 TeV source, with a data sample collected with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 3.2 pb^(â1). Event-shape distributions, corrected for detector response, are compared with five models of QCD multijet production
Search for Three-Jet Resonances in pp Collisions at âs = 7 TeV
A search for three-jet hadronic resonance production in pp collisions at a center-of-mass energy of
7 TeV has been conducted by the CMS Collaboration at the LHC, using a data sample corresponding to an
integrated luminosity of 35 pb^(-1). Events with high jet multiplicity and a large scalar sum of jet transverse
momenta are analyzed using a signature-based approach. The number of expected standard model
background events is found to be in good agreement with the observed events. Limits on the cross
section times branching ratio are set in a model of gluino pair production with an R-parity-violating decay
to three quarks, and the data rule out such particles within the mass range of 200 to 280 GeV/c^2
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