Event generator tunes obtained from underlying event and multiparton scattering measurements

New sets of parameters (“tunes”) for the underlying-event (UE) modelling of the PYTHIA8, PYTHIA6 and HERWIG++ MonteCarlo event generators are constructed using different parton distribution functions. Combined fits to CMS UE proton–proton (pp) data at √s = 7 TeV 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 13 TeV. 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 8 TeV, as well as predictions for MB and UE observables at 13 TeV

Search for charge-asymmetric production of W\u27 bosons in tt + jet events from pp collisions at √s = 7 TeV

A search is presented for charge-asymmetric production of a W\u27 boson that has been proposed to accommodate the forward–backward asymmetry observed in the production of top–antitop quark pairs at the Tevatron. The new heavy W\u27 boson would be produced in association with a top quark and would decay into top and down quarks. The data correspond to an integrated luminosity of 5.0 fb−1 in pp collisions at a center-of-mass energy of 7 TeV, recorded by the CMS detector at the LHC. No significant excess above the standard model expectations is observed, and, from a combination of the electron-plusjets and muon-plus-jets channels, a 95% confidence level lower limit of 840 GeV/c2 is set on the W\u27 boson mass for a W\u27 boson model with values for coupling constants to top and down quarks gL = 0 and gR = 2. In addition, a kinematic reconstruction of the W[1] resonance mass using the inherent charge asymmetry of this model finds no indication of the presence of W\u27 events in the data

Search for physics beyond the standard model in events with a Z boson, jets, and missing transverse energy in pp collisions at √s = 7 TeV

A search is presented for physics beyond the standard model (BSM) in events with a Z boson, jets, and missing transverse energy (EmissT ). This signature is motivated by BSM physics scenarios, including supersymmetry. The study is performed using a sample of proton–proton collision data collected at √s = 7 TeV with the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.98 fb−1. The contributions from the dominant standard model backgrounds are estimated from data using two complementary strategies, the jet-Z balance technique and a method based on modeling EmissT with data control samples. In the absence of evidence for BSM physics, we set limits on the non-standard-model contributions to event yields in the signal regions and interpret the results in the context of simplified model spectra. Additional information is provided to facilitate tests of other BSM physics models

Jet momentum dependence of jet quenching in PbPb collisions at √sNN = 2.76 TeV

Dijet production in PbPb collisions at a nucleon–nucleon center-of-mass energy of 2.76 TeV is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 150 μb−1 is analyzed. Jets are reconstructed using combined information from tracking and calorimetry, using the anti-kT algorithm with R = 0.3. The dijet momentum balance and angular correlations are studied as a function of collision centrality and leading jet transverse momentum. For the most peripheral PbPb collisions, good agreement of the dijet momentum balance distributions with pp data and reference calculations at the same collision energy is found, while more central collisions show a strong imbalance of leading and subleading jet transverse momenta attributed to the jet-quenching effect. The dijets in central collisions are found to be more unbalanced than the reference, for leading jet transverse momenta up to the highest values studied

Search for narrow resonances in dilepton mass spectra in pp collisions at √s = 7 TeV

A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at √s = 7 TeV by the CMS experiment in 2011. The data sample corresponds to an integrated luminosity of approximately 5 fb−1. The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the Z\u27 in the Sequential Standard Model, 2000 GeV for the superstring-inspired Z\u27ψ resonance, 890 (540) GeV for the Stueckelberg extension Z\u27St with the mass parameter є = 0.06 (0.04), and 2140 (1810) GeV for Kaluza–Klein gravitons with the coupling parameter k/M̅Pl of 0.10 (0.05). These limits are the most stringent to date

Search for neutral Higgs bosons decaying to tau pairs in pp collisions at √s = 7 TeV

A search for neutral Higgs bosons decaying to tau pairs at a center-of-mass energy of 7 TeV is performed using a dataset corresponding to an integrated luminosity of 4.6 fb−1 recorded by the CMS experiment at the LHC. The search is sensitive to both the standard model Higgs boson and to the neutral Higgs bosons predicted by the minimal super symmetric extension of the standard model (MSSM). No excess of events is observed in the tau-pair invariant-mass spectrum. For a standard model Higgs boson in the mass range of 110–145 GeV upper limits at 95% confidence level (CL) on the production cross section are determined. We exclude a Higgs boson with mH = 115 GeV with a production cross section 3.2 times of that predicted by the standard model. In the MSSM, upper limits on the neutral Higgs boson production cross section times branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, mA, sets stringent new bounds in the parameter space, excluding at 95% CL values of tan β as low as 7.1 at mA = 160 GeV in the mmaxh benchmark scenario

Search for heavy, top-like quark pair production in the dilepton final state in pp collisions at √s = 7 TeV

The results of a search for pair production of a heavy, top-like quark, t\u27, in the decay mode t\u27t̅\u27 → bW+ b̅W− → bℓ +νb̅ℓ −ν̅ are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0 fb−1 in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t\u27t̅\u27 production is found. Upper limits on the production cross section as a function of t\u27 mass are presented, and t\u27 masses below 557 GeV/c2 are excluded at the 95% confidence level

Measurement of the Λb cross section and the Λb to Λb ratio with J/ψΛ decays in pp collisions at √s = 7 TeV

The Λb differential production cross section and the cross section ratio σ(Λ̅b )/σ(Λb) are measured as functions of transverse momentum pΛbT and rapidity |yΛb | in pp collisions at √ s = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on Λb decays reconstructed in the exclusive final state J/ψΛ, with the subsequent decays J/ψ →μ+μ− and Λ→pπ, using a data sample corresponding to an integrated luminosity of 1.9 fb−1. The product σ(Λb) × B(Λb →J/ψΛ) versus pΛbT falls faster than that of b mesons. The measured value of σ(Λb) × B(Λb →J/ψΛ) for pΛbT \u3e 10 GeV and |yΛb | \u3c 2.0 is 1.16±0.06±0.12 nb, and the integrated σ(Λ̅b)/σ(Λb) ratio is 1.02±0.07±0.09, where the uncertainties are statistical and systematic, respectively

Study of the production of charged pions, kaons, and protons in pPb collisions at √sNN = 5.02TeV

Spectra of identified charged hadrons are meas√ ured in pPb collisions with the CMS detector at the LHC at sNN = 5.02 TeV. Charged pions, kaons, and protons in the transverse-momentum range pT ≈ 0.1–1.7 GeV/c and laboratory rapidity |y| \u3c 1 are identified via their energy loss in the silicon tracker. The average pT increases with particle mass and the charged multiplicity of the event. The increase of the average pT with charged multiplicity is greater for heavier hadrons. Comparisons to Monte Carlo event generators reveal that Epos Lhc, which incorporates additional hydrodynamic evolution of the created system, is able to reproduce most of the data features, unlike Hijing and Ampt. The pT spectra and integrated yields are also compared to those measured in pp and PbPb collisions at various energies. The average transverse momentum and particle ratio measurements indicate that particle production at LHC energies is strongly correlated with event particle multiplicity

Study of double parton scattering using W + 2-jet events in proton-proton collisions at √s = 7 TeV

Double parton scattering is investigated in proton-proton collisions at √s = 7 TeV where the final state includes a W boson, which decays into a muon and a neutrino, and two jets. The data sample corresponds to an integrated luminosity of 5 fb−1, collected with the CMS detector at the LHC. Observables sensitive to double parton scattering are investigated after being corrected for detector effects and selection efficiencies. The fraction of W + 2-jet events due to double parton scattering is measured to be 0.055 ± 0.002 (stat.) ± 0.014 (syst.). The effective cross section, σeff, characterizing the effective transverse area of hard partonic interactions in collisions between protons is measured to be 20.7 ± 0.8 (stat.) ± 6.6 (syst.)mb