2,122 research outputs found

    Measurement of Leptonic Asymmetries and Top Quark Polarization in ttbar Production

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    We present measurements of lepton (l) angular distributions in ttbar -> W+ b W- b -> l+ nu b l- nubar bbar decays produced in ppbar collisions at a center-of-mass energy of sqrt(s)=1.96TeV, where l is an electron or muon. Using data corresponding to an integrated luminosity of 5.4fb^-1, collected with the D0 detector at the Fermilab Collider, we find that the angular distributions of l- relative to anti-protons and l+ relative to protons are in agreement with each other. Combining the two distributions and correcting for detector acceptance we obtain the forward-backward asymmetry A^l_FB = (5.8 +- 5.1(stat) +- 1.3(syst))%, compared to the standard model prediction of A^l_FB (predicted) = (4.7 +- 0.1)%. This result is further combined with the measurement based on the analysis of the l+jets final state to obtain A^l_FB = (11.8 +- 3.2)%. Furthermore, we present a first study of the top-quark polarization.Comment: submitted versio

    Search for Zgamma events with large missing transverse energy in ppbar collisions at sqrt(s)=1.96 TeV

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    We present the first search for supersymmetry (SUSY) in Zgamma final states with large missing transverse energy using data corresponding to an integrated luminosity of 6.2 fb-1 collected with the D0 experiment in ppbar collisions at sqrt(s)=1.96 TeV. This signature is predicted in gauge-mediated SUSY-breaking models, where the lightest neutralino is the next-to-lightest supersymmetric particle (NLSP) and is produced in pairs, possibly through decay from heavier supersymmetric particles. The NLSP can decay either to a Z boson or a photon and an associated gravitino that escapes detection. We exclude this model at the 95% C.L. for SUSY breaking scales of Lambda < 87 TeV, corresponding to neutralino masses of < 151 GeV.Comment: submitted to Phys. Rev. Let

    Measurements of single top quark production cross sections and |Vtb| in ppbar collisions at sqrt{s}=1.96 TeV

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    We present measurements of production cross sections of single top quarks in \ppbar collisions at s=1.96  TeV\sqrt{s}=1.96\;\rm TeV in a data sample corresponding to an integrated luminosity of 5.4  fb−15.4\;\rm fb^{-1} collected by the D0 detector at the Fermilab Tevatron Collider. We select events with an isolated electron or muon, an imbalance in transverse energy, and two, three, or four jets, with one or two of them containing a bottom hadron. We obtain an inclusive cross section of \sigma({\ppbar}{\rargap}tb+X, tqb+X) = 3.43\pm^{0.73}_{0.74}\;\rm pb and use it to extract the CKM matrix element 0.79<∣VtbâˆŁâ‰€10.79 < |V_{tb}| \leq 1 at the 95% C.L. We also measure \sigma({\ppbar}{\rargap}tb+X) = 0.68\pm^{0.38}_{0.35}\;\rm pb and \sigma({\ppbar}{\rargap}tqb+X) = 2.86\pm^{0.69}_{0.63}\;\rm pb when assuming, respectively, tqbtqb and tbtb production rates as predicted by the standard model.Comment: 11 pages, 8 figures, submitted to Phys. Rev.

    Zgamma production and limits on anomalous ZZgamma and Zgammagamma couplings in ppbar collisions at sqrt(s)=1.96 TeV

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    We present a measurement of ppbar->Zgamma->ll+gamma (l = e, mu) production with a data sample corresponding to an integrated luminosity of 6.2 fb^{-1} collected by the D0 detector at the Fermilab Tevatron ppbar Collider. The results of the electron and muon channels are combined, and we measure the total production cross section and the differential cross section dsigma/dp_T^gamma, where p_T^gamma is the momentum of the photon in the plane transverse to the beamline. The results obtained are consistent with the standard model predictions from next-to-leading order calculations. We use the transverse momentum spectrum of the photon to place limits on anomalous ZZgamma and Zgammagamma couplings

    Precise measurement of the top quark mass in the dilepton channel at D0

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    We measure the top quark mass (mt) in ppbar collisions at a center of mass energy of 1.96 TeV using dilepton ttbar->W+bW-bbar->l+nubl-nubarbbar events, where l denotes an electron, a muon, or a tau that decays leptonically. The data correspond to an integrated luminosity of 5.4 fb-1 collected with the D0 detector at the Fermilab Tevatron Collider. We obtain mt = 174.0 +- 1.8(stat) +- 2.4(syst) GeV, which is in agreement with the current world average mt = 173.3 +- 1.1 GeV. This is currently the most precise measurement of mt in the dilepton channel.Comment: 7 pages, 4 figure

    Direct measurement of the mass difference between top and antitop quarks

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    We present a direct measurement of the mass difference between top and antitop quarks (dm) in lepton+jets top-antitop final states using the "matrix element" method. The purity of the lepton+jets sample is enhanced for top-antitop events by identifying at least one of the jet as originating from a b quark. The analyzed data correspond to 3.6 fb-1 of proton-antiproton collisions at 1.96 TeV acquired by D0 in Run II of the Fermilab Tevatron Collider. The combination of the e+jets and mu+jets channels yields dm = 0.8 +/- 1.8 (stat) +/- 0.5 (syst) GeV, which is in agreement with the standard model expectation of no mass difference.Comment: submitted to Phys. Rev.

    Measurement of the semileptonic charge asymmetry using Bs^0 -> D_s mu X decays

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    We present a measurement of the time-integrated flavor-specific semileptonic charge asymmetry in the decays of Bs mesons that have undergone flavor mixing, a^s_ls, using B_s^0 (barB_s^0) -> D_s^-/+ mu^+/- X decays, with D_s^-/+ -> phi pi^-/+ and phi K^+ K^-, using 10.4 fb^-1 of proton-antiproton collisions collected by the D0 detector during Run II at the Fermilab Tevatron Collider. A fit to the difference between the time-integrated D_s^-$ and D_s^+ mass distributions of the B_s^0 and barB^0_s candidates yields the flavor-specific asymmetry a^s_ls = [-1.12 pm 0.74 (stat) +/-0.17 (syst)]% which is the most precise measurement and in agreement with the standard model prediction.Comment: 7 pages, 3 figures (updated with changes made in journal review process

    Search for the standard model and a fermiophobic Higgs boson in diphoton final states

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    We present a search for the standard model Higgs boson and a fermiophobic Higgs boson in the diphoton final states based on 8.2 fb-1 of ppbar collisions collected with the D0 detector at the Fermilab Tevatron Collider. No excess of data above background predictions is observed and upper limits at the 95% C.L. on the cross section multiplied by the branching fraction are set which are the most restrictive to date. A fermiophobic Higgs boson with a mass below 112.9 GeV is excluded at the 95% C.L.Comment: 12 pages, 21 figures, submitted to PR

    Measurement of the top quark pair production cross section in the lepton+jets channel in proton-antiproton collisions at s\sqrt{s}=1.96 TeV

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    We present a measurement of the inclusive top quark pair production cross section in \ppbar collisions at (\sqrt{s}=1.96) TeV utilizing data corresponding to an integrated luminosity of \lumi\ collected with the D0 detector at the Fermilab Tevatron Collider. We consider final states containing one high-pTp_{T} isolated electron or muon and at least two jets, and we perform three analyses: one exploiting specific kinematic features of \ttbar events, the second using bb-jet identification, and the third using both techniques to separate \ttbar\ signal from background. In the third case, we determine simultaneously the ttˉt\bar{t} cross section and the ratio of the production rates of WW+heavy flavor jets and WW+light flavor jets, which reduces the impact of the systematic uncertainties related to the background estimation. Assuming a top quark mass of 172.5 GeV, we obtain σttˉ=7.78−0.64+0.77\sigma_{t\bar{t}} = 7.78^{+0.77}_{-0.64} pb. This result agrees with predictions of the standard model.Comment: 19 pages, 6 figures, submitted to Phys. Rev.