16 research outputs found

    Description and performance of track and primary-vertex reconstruction with the CMS tracker

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    A description is provided of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices. Despite the very hostile environment at the LHC, the performance obtained with these algorithms is found to be excellent. For t (t) over bar events under typical 2011 pileup conditions, the average track-reconstruction efficiency for promptly-produced charged particles with transverse momenta of p(T) > 0.9GeV is 94% for pseudorapidities of vertical bar eta vertical bar < 0.9 and 85% for 0.9 < vertical bar eta vertical bar < 2.5. The inefficiency is caused mainly by hadrons that undergo nuclear interactions in the tracker material. For isolated muons, the corresponding efficiencies are essentially 100%. For isolated muons of p(T) = 100GeV emitted at vertical bar eta vertical bar < 1.4, the resolutions are approximately 2.8% in p(T), and respectively, 10 m m and 30 mu m in the transverse and longitudinal impact parameters. The position resolution achieved for reconstructed primary vertices that correspond to interesting pp collisions is 10-12 mu m in each of the three spatial dimensions. The tracking and vertexing software is fast and flexible, and easily adaptable to other functions, such as fast tracking for the trigger, or dedicated tracking for electrons that takes into account bremsstrahlung

    Search for pair production of third-generation scalar leptoquarks and top squarks in proton-proton collisions at v root s=8 TeV

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    A search for pair production of third-generation scalar leptoquarks and supersymmetric top quark partners, top squarks, in final states involving tau leptons and bottom quarks is presented. The search uses events from a data sample of proton-proton collisions corresponding to an integrated luminosity of 19.7 fb(-1), collected with the CMS detector at the LHC with root s = 8 TeV. The number of observed events is found to be in agreement with the expected standard model background. Third-generation scalar leptoquarks with masses below 740 GeV are excluded at 95% confidence level, assuming a 100% branching fraction for the leptoquark decay to a tau lepton and a bottom quark. In addition, this mass limit applies directly to top squarks decaying via an R-parity violating coupling. lambda(') (333). The search also considers a similar signature from top squarks undergoing a chargino-mediated decay involving the Rparity violating coupling. lambda(')(3jk). Each top squark decays to a tau lepton, a bottom quark, and two light quarks. Top squarks in this model with masses below 580 GeV are excluded at 95% confidence level. The constraint on the leptoquark mass is the most stringent to date, and this is the first search for top squarks decaying via. lambda(')(3jk)

    Performance of the CMS missing transverse momentum reconstruction in pp data at √s= 8 TeV

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    The performance of missing transverse energy reconstruction algorithms is presented using root s = 8 TeV proton-proton (pp) data collected with the CMS detector. Events with anomalous missing transverse energy are studied, and the performance of algorithms used to identify and remove these events is presented. The scale and resolution for missing transverse energy, including the effects of multiple pp interactions (pileup), are measured using events with an identified Z boson or isolated photon, and are found to be well described by the simulation. Novel missing transverse energy reconstruction algorithms developed specifically to mitigate the effects of large numbers of pileup interactions on the missing transverse energy resolution are presented. These algorithms significantly reduce the dependence of the missing transverse energy resolution on pileup interactions. Finally, an algorithm that provides an estimate of the significance of the missing transverse energy is presented, which is used to estimate the compatibility of the reconstructed missing transverse energy with a zero nominal value

    Constraints on the Higgs boson width from off-shell production and decay to Z-boson pairs

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    Constraints 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 fb(-1) at a center-of-mass energy root s = 7 TeV and 19.7 fb(-1) at root 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 of m(H) = 125.6 GeV

    Measurement of the production cross section ratio sigma(chi b2(1P))/sigma(chi b1(1P)) in pp collisions at root s=8TeV

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    A measurement of the production cross section ratio sigma(chi b2(1P))/sigma(chi b1(1P)) is presented. The chi b1 (1P) and chi b2 (1P) bottomonium states, promptly produced in pp collisions at root s = 8TeV, are detected by the CMS experiment at the CERN LHC through their radiative decays chi b1,2(1P) -> Y(1S) + gamma. The emitted photons are measured through their conversion to e(+) e(-) pairs, whose reconstruction allows the two states to be resolved. The Y(1S) is measured through its decay to two muons. An event sample corresponding to an integrated luminosity of 20.7 fb(-1) is used to measure the cross section ratio in a phase-space region defined by the photon pseudorapidity, vertical bar eta(gamma)vertical bar < 1.0; the Y(1S) rapidity, vertical bar y(Y)vertical bar < 1.5; and the Y(1S) transverse momentum, 7 < p(T)(Y) < 40 GeV. The cross section ratio shows no significant dependence on the.(1S) transverse momentum, with a measured average value of 0.85 +/- 0.07 (stat + syst) +/- 0.08 (BF), where the first uncertainty is the combination of the experimental statistical and systematic uncertainties and the second is from the uncertainty in the ratio of the chi b branching fractions

    Search for top-squark pairs decaying into Higgs or Z bosons in pp collisions at root s=8 TeV

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    A search for supersymmetry through the direct pair production of top squarks, with Higgs (H) or Z bosons in the decay chain, is performed using a data sample of proton-proton collisions at root s = 8 TeVcollected in 2012 with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 19.5 fb(-1). The search is performed using a selection of events containing leptons and bottom-quark jets. No evidence for a significant excess of events over the standard model background prediction is observed. The results are interpreted in the context of simplified supersymmetric models with pair production of a heavier top-squark mass eigenstate (t) over tilde (2) decaying to a lighter top-squark mass eigenstate (t) over tilde (1) via either (t) over tilde (2) -> H (t) over tilde (1) or (t) over tilde (2) -> Z (t) over tilde (1), followed in both cases by (t) over tilde (1) -> t (chi) over tilde (0)(1), where (chi) over tilde (0)(1) is an undetected, stable, lightest supersymmetric particle. The interpretation is performed in the region where the mass difference between the (t) over tilde (1) and (chi) over tilde (0)(1) states is approximately equal to the top-quark mass (m((t) over tilde )1) - m((chi) over tilde 10) similar or equal to m(t)) which is not probed by searches for direct (t) over tilde (1) squark pair production. The analysis excludes top squarks with masses m((t) over tilde2) 1) < 400 GeVat a 95% confidence level

    Measurement of the ratio B(t -> Wb)/B(t -> Wq) in pp collisions at root s=8 TeV

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    The ratio of the top-quark branching fractions R = B(t --> Wb)/B(t --> Wq), where the denominator includes the sum over all down-type quarks (q = b, s, d), is measured in the t (t) over bar dilepton final state with proton-proton collision data at root s = 8 TeV from an integrated luminosity of 19.7 fb(-1), collected with the CMS detector. In order to quantify the purity of the signal sample, the cross section is measured by fitting the observed jet multiplicity, thereby constraining the signal and background contributions. By counting the number of b jets per event, an unconstrained value of R = 1.014 +/- 0.003 (stat.) +/- 0.032 (syst.) is measured, in a good agreement with current precision measurements in electroweak and flavour sectors. A lower limit R > 0.955 at the 95% confidence level is obtained after requiring R 0.975 is set at 95% confidence level. The result is combined with a previous CMS measurement of the t-channel single-top-quark cross section to determine the top-quark total decay width, Gamma(t) = 1.36 +/- 0.02 (stat.)(-0.11)(+0.14) (syst.) GeV

    Measurement of the production cross section for a W boson and two b jets in pp collisions at root s=7 TeV

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    The production cross section for a Wboson and two b jets is measured using proton-proton collisions at v root s = 7 TeV in a data sample collected with the CMS experiment at the LHC corresponding to an integrated luminosity of 5.0 fb(-1). The W + bbevents are selected in the W..decay mode by requiring a muon with transverse momentum pT> 25GeVand pseudorapidity |eta| 25GeVand |eta| < 2.4. The measured W + bbproduction cross section in the fiducial region, calculated at the level of final-state particles, is s(pp. W + bb) xB(W..) = 0.53 +/- 0.05 (stat.) +/- 0.09 (syst.) +/- 0.06 (theo.) 0.01 (lum.) pb, in agreement with the standard model prediction. In addition, kinematic distributions of the W + bbsystem are in agreement with the predictions of a simulation usingMadGraphandpythia

    Search for a standard model-like Higgs boson in the mu(+)mu(-) and e(+)e(-) decay channels at the LHC

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    A search is presented for a standard model-like Higgs boson decaying to the mu(+)mu(-) or e(+)e(-) final states based on proton-proton collisions recorded by the CMS experiment at the CERN LHC. The data correspond to integrated luminosities of 5.0 fb(-1) at a centre-of-mass energy of 7 TeV and 19.7 fb(-1) at 8 TeV for the mu(+)mu(-) search, and of 19.7 fb(-1) at 8 TeV for the e(+)e(-) search. Upper limits on the production cross section times branching fraction at the 95% confidence level are reported for Higgs boson masses in the range from 120 to 150 GeV. For a Higgs boson with a mass of 125 GeV decaying to mu(+)mu(-), the observed (expected) upper limit on the production rate is found to be 7.4 (6.5(-1.9)(+2.8)) times the standard model value. This corresponds to an upper limit on the branching fraction of 0.0016. Similarly, for e(+)e(-), an upper limit of 0.0019 is placed on the branching fraction, which is approximate to 3.7 x 10(5) times the standard model value. These results, together with recent evidence of the 125 GeV boson coupling to tau-leptons with a larger branching fraction consistent with the standard model, confirm that the leptonic couplings of the new boson are not flavour-universal

    Measurement of the t(t)over-bar production cross section in pp collisions at root s=8 TeV in dilepton final states containing one tau lepton

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    The top-quark pair production cross section is measured in final states with one electron or muon and one hadronically decaying tau lepton from the process t (t) over bar -> (l nu(l))(tau nu(tau))b (b) over bar, where l = e, mu. The data sample corresponds to an integrated luminosity of 19.6 fb(-1) collected with the CMS detector in proton-proton collisions at root s = 8 TeV. The measured cross section sigma(t (t) over bar) = 257 +/- 3 (stat) +/- 24 (syst) +/- 7 (lumi) pb, assuming a top-quark mass of 172.5 GeV, is consistent with the standard model prediction
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