6,347 research outputs found

    Experimental methods in top physics

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    Top quark physics at the Tevatron has inspired particle physicists to develop new methods of understanding hadron collider data. From sophisticated one-dimensional fitting techniques to multidimensional neural networks, the top quark physics program has innovated many techniques which at this time are used extensively in the high-pT program at the Tevatron and LHC

    Measurement of the cross section for direct-photon production in association with a heavy quark in pp¯ collisions at s√ = 1.96 TeV

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    We report on a measurement of the cross section for direct-photon production in association with a heavy quark using the full data set of s√=1.96  TeV proton-antiproton collisions corresponding to 9.1  fb−1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron. The measurements are performed as a function of the photon transverse momentum, covering a photon transverse momentum between 30 and 300 GeV, photon rapidities |yγ|20  GeV, and jet rapidities |yjet|<1.5. The results are compared with several theoretical predictions

    Indirect measurement of sin2θW\sin^2 \theta_W (or MWM_W) using μ+μ\mu^+\mu^- pairs from γ/Z\gamma^*/Z bosons produced in ppˉp\bar{p} collisions at a center-of-momentum energy of 1.96 TeV

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    Drell-Yan lepton pairs are produced in the process ppˉμ+μ+Xp\bar{p} \rightarrow \mu^+\mu^- + X through an intermediate γ/Z\gamma^*/Z boson. The forward-backward asymmetry in the polar-angle distribution of the μ\mu^- as a function of the invariant mass of the μ+μ\mu^+\mu^- pair is used to obtain the effective leptonic determination sin2θefflept\sin^2 \theta^{lept}_{eff} of the electroweak-mixing parameter sin2θW\sin^2 \theta_W, from which the value of sin2θW\sin^2 \theta_W is derived assuming the standard model. The measurement sample, recorded by the Collider Detector at Fermilab (CDF), corresponds to 9.2 fb-1 of integrated luminosity from ppˉp\bar{p} collisions at a center-of-momentum energy of 1.96 TeV, and is the full CDF Run II data set. The value of sin2θefflept\sin^2 \theta^{lept}_{eff} is found to be 0.2315 +- 0.0010, where statistical and systematic uncertainties are combined in quadrature. When interpreted within the context of the standard model using the on-shell renormalization scheme, where sin2θW=1MW2/MZ2\sin^2 \theta_W = 1 - M_W^2/M_Z^2, the measurement yields sin2θW\sin^2 \theta_W = 0.2233 +- 0.0009, or equivalently a W-boson mass of 80.365 +- 0.047 GeV/c^2. The value of the W-boson mass is in agreement with previous determinations in electron-positron collisions and at the Tevatron collider

    Forward-backward asymmetry of Drell–Yan lepton pairs in pp collisions at s\sqrt{s} = 8 TeV

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    A measurement of the forward-backward asymmetry AFB of oppositely charged lepton pairs (μμ and ee) produced via Z/γ∗ boson exchange in pp collisions at s\sqrt{s} = 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.7 fb1^{−1} collected with the CMS detector at the LHC. The measurement of AFBA_{FB} is performed for dilepton masses between 40 GeV and 2 TeV and for dilepton rapidity up to 5. The AFBA_{FB} measurements as a function of dilepton mass and rapidity are compared with the standard model predictions

    Measurement of the CP-violating weak phase ϕsϕ_s and the decay width difference ΔΓsΔΓ_s using the Bs0J/ψϕ(1020)B^{0}_{s}→J/ψϕ(1020) decay channel in pp collisions at s=8\sqrt{s} = 8 TeV

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    The CP-violating weak phase ϕsϕ_s of the Bs0B^{0}_{s} meson and the decay width difference ΔΓsΔΓ_s of the Bs0 light and heavy mass eigenstates are measured with the CMS detector at the LHC using a data sample of Bs0J/ψϕ(1020)μ+μK+KB^{0}_{s}→J/ψϕ(1020)→μ^{+}μ^{−}K^{+}K^{−} decays. The analysed data set corresponds to an integrated luminosity of 19.7fb1^{−1} collected in pp collisions at a centre-of-mass energy of 8TeV . A total of 49 200 reconstructed Bs0B^{0}_{s} decays are used to extract the values of ϕsϕ_s and ΔΓsΔΓ_s by performing a time-dependent and flavour-tagged angular analysis of the μ+μK+Kμ^{+}μ^{−}K^{+}K^{−} final state. The weak phase is measured to be ϕsϕ_s=−0.075±0.097(stat)±0.031(syst) rad , and the decay width difference is ΔΓsΔΓ_s=0.095±0.013(stat)±0.007(syst) ps1^{−1}

    Measurements of ttt\overline{t} spin correlations and top quark polarization using dilepton final states in pp collisions at s\sqrt{s} = 8 TeV

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    Measurements of the top quark-antiquark (ttt\overline{t}) spin correlations and the top quark polarization are presented for ttt\overline{t} pairs produced in pp collisions at s\sqrt{s}=8  TeV. The data correspond to an integrated luminosity of 19.5  fb1^{−1} collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the ttt\overline{t} system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated

    Search for supersymmetry in electroweak production with photons and large missing transverse energy in pp collisions at s=8TeV\sqrt{s} = 8\mathrm{\,Te\kern -0.1em V}

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    Results are reported from a search for supersymmetry with gauge-mediated supersymmetry breaking in electroweak production. Final states with photons and large missing transverse energy ( ETmissE^{miss}_{T} ) were examined. The data sample was collected in pp collisions at s\sqrt{s} =8TeV with the CMS detector at the LHC and corresponds to 7.4 fb1^{−1} . The analysis focuses on scenarios in which the lightest neutralino has bino- or wino-like components, resulting in decays to photons and gravitinos, where the gravitinos escape undetected. The data were obtained using a specially designed trigger with dedicated low thresholds, providing good sensitivity to signatures with photons, ETmissE^{miss}_{T} , and low hadronic energy. No excess of events over the standard model expectation is observed. The results are interpreted using the model of general gauge mediation. With the wino mass fixed at 10 GeVGeV above that of the bino, wino masses below 710 GeVGeV are excluded at 95% confidence level. Constraints are also set in the context of two simplified models, for which the analysis sets the lowest cross section limits on the electroweak production of supersymmetric particles

    Search for lepton flavour violating decays of the Higgs boson to eττ and eμ in proton–proton collisions at s\sqrt{s} = 8 TeV

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    A direct search for lepton flavour violating decays of the Higgs boson (H) in the H→eτ and H→eμ channels is described. The data sample used in the search was collected in proton–proton collisions at s\sqrt{s}=8 TeV with the CMS detector at the LHC and corresponds to an integrated luminosity of 19.7 fb1^{−1}. No evidence is found for lepton flavour violating decays in either final state. Upper limits on the branching fractions, B(H→eτ)<0.69% and B(H→eμ)<0.035% , are set at the 95% confidence level. The constraint set on B(H→eτ) is an order of magnitude more stringent than the existing indirect limits. The limits are used to constrain the corresponding flavour violating Yukawa couplings, absent in the standard model

    Measurement of the ZγννγZγ → ν\overline{ν}γ production cross section in pp collisions at s\sqrt{s} = 8 TeV and limits on anomalous ZZγ and Zγγ trilinear gauge boson couplings

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    An inclusive measurement of the ZγννγZγ → ν\overline{ν}γ production cross section in pp collisions at s\sqrt{s}=8TeV is presented, using data corresponding to an integrated luminosity of 19.6 fb1^{−1} collected with the CMS detector at the LHC. This measurement is based on the observation of events with large missing energy and with a single photon with transverse momentum above 145 GeV and absolute pseudorapidity in the range |η|<1.44 . The measured Zγ→νν‾γ production cross section, 52.7±2.1 (stat)±6.4 (syst)±1.4 (lumi) fb , agrees well with the standard model prediction of 50.0−2.2+2.4 fb . A study of the photon transverse momentum spectrum yields the most stringent limits to date on the anomalous ZZγ and Zγγ trilinear gauge boson couplings

    Measurements of WγW\gamma and ZγZ\gamma production in pppp collisions at s\sqrt{s} =7  TeV with the ATLAS detector at the LHC

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    The integrated and differential fiducial cross sections for the production of a WW or ZZ boson in association with a high-energy photon are measured using pppp collisions at s\sqrt{s} = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6fb14.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the WW and ZZ bosons [W(eν,μν][W(e \nu,\mu \nu] and Z(e+e,μ+μ,ννˉ))Z(e^+ e^-, \mu^+ \mu^-, \nu \bar{\nu})) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWγWW\gamma, ZZγZZ\gamma and ZγγZ\gamma\gamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to ZγZ\gamma and WγW\gamma. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances
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