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Publications (57)0 Total impact

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    D0 Collaboration, V. Abazov
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    ABSTRACT: We report a search for diphoton events with large missing transverse energy produced in ppbar collisions at sqrt(s)=1.96 TeV. The data were collected with the D0 detector at the Fermilab Tevatron Collider, and correspond to 6.3 fb-1 of integrated luminosity. The observed missing transverse energy distribution is well described by the standard model prediction, and 95% C.L. limits are derived on two realizations of theories beyond the standard model. In a gauge mediated supersymmetry breaking scenario, the breaking scale Lambda is excluded for Lambda < 124 TeV. In a universal extra dimension model including gravitational decays, the compactification radius R_c is excluded for R_c-1 < 477 GeV. Comment: Submitted to Phys. Rev. Lett
    08/2010;
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    D0 Collaboration, V. M. Abazov
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    ABSTRACT: We report results from a search for production of a neutral Higgs boson in association with a $b$ quark. We search for Higgs decays to $\tau$ pairs with one $\tau$ subsequently decaying to a muon and the other to hadrons. The data correspond to 2.7fb$^{-1}$ of $\ppbar$ collisions recorded by the D0 detector at $\sqrt{s} = 1.96$TeV. The data are found to be consistent with background predictions. The result allows us to exclude a significant region of parameter space of the minimal supersymmetric model. Comment: Submitted to Phys. Rev. Letters
    12/2009;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present a direct measurement of the width of the W boson using the shape of the transverse mass distribution of W->enu candidates selected in 1 fb-1 of data collected with the D0 detector at the Fermilab Tevatron collider in ppbar collisions at sqrt{s}=1.96 TeV. We use the same methods and data sample that were used for our recently published W boson mass measurement, except for the modeling of the recoil, which is done with a new method based on a recoil library. Our result, 2.028 +- 0.072 GeV, is in agreement with the predictions of the standard model. Comment: 7 pages, 1 figure, published in Phys. Rev. Lett
    09/2009;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present a measurement of the mass difference between t and tbar quarks observed in lepton+jets final states of ttbar events in 1 fb-1 of data collected with the D0 detector at the Fermilab Tevatron Collider in ppbar collisions at sqrt(s)=1.96 TeV. The observed mass difference of 3.8+-3.7 GeV is consistent with the equality of t and tbar masses. This is the first direct measurement of a mass difference between a quark and its antiquark partner. Comment: submitted to Phys. Rev. Letters
    06/2009;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present a search for the standard model Higgs boson using hadronically decaying tau leptons, in 1 inverse femtobarn of data collected with the D0 detector at the Fermilab Tevatron ppbar collider. We select two final states: tau plus missing transverse energy and b jets, and tau+ tau- plus jets. These final states are sensitive to a combination of associated W/Z boson plus Higgs boson, vector boson fusion and gluon-gluon fusion production processes. The observed ratio of the combined limit on the Higgs production cross section at the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of 115 GeV. Comment: publication version
    03/2009;
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    D0 Collaboration, V. M. Abazov
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    ABSTRACT: We present the first evidence of WW+WZ production with lepton+jets final states at a hadron collider. The data correspond to 1.07 inverse femtobarns of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider in proton-antiproton collisions at sqrt(s)=1.96 TeV. The observed cross section for WW+WZ production is 20.2 +/- 4.5 pb, consistent with the SM prediction of 16.1 +/- 0.9 pb. The probability for background fluctuations to produce an excess equal to or larger than that observed is estimated to be 5.4e-6, corresponding to a significance of 4.4 standard deviations. Comment: 11 pages, 4 figures, 3 tables, published in Physical Review Letters, updated to published version
    10/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, like hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark.
    07/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We describe a search for production of a charged Higgs boson, q \bar{q'} -> H^+, reconstructed in the t\bar{b} final state in the mass range 180 <= M_{H^+} <= 300 GeV. The search was undertaken at the Fermilab Tevatron collider with a center-of-mass energy sqrt{s} = 1.96 TeV and uses 0.9 fb^{-1} of data collected with the D0 detector. We find no evidence for charged Higgs boson production and set upper limits on the production cross section in the Types I, II and III two-Higgs-doublet models (2HDMs). An excluded region in the (M_{H^+},tan\beta) plane for Type I 2HDM is presented. Comment: Submitted to Phys. Rev. Letters
    07/2008;
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    D0 Collaboration, V. M. Abazov
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    ABSTRACT: We present a measurement of the fraction of inclusive $W$+jets events produced with net charm quantum number $\pm1$, denoted $W$+$c$-jet, in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV using approximately 1~fb$^{-1}$ of data collected by the D0 detector at the Fermilab Tevatron Collider. We identify the $W$+jets events via the leptonic $W$ boson decays. Candidate $W$+$c$-jet events are selected by requiring a jet containing a muon in association with a reconstructed $W$ boson and exploiting the charge correlation between this muon and $W$ boson decay lepton to perform a nearly model-independent background subtraction. We measure the fraction of $W$+$c$-jet events in the inclusive $W$+jets sample for jet $p_{T}>20$ GeV and pseudorapidity $|\eta|<2.5$ to be 0.074$\pm0.019$(stat.)$\pm^{0.012}_{0.014}$(syst.), in agreement with theoretical predictions. The probability that background fluctuations could produce the observed fraction of $W$+$c$-jet events is estimated to be $2.5\times 10^{-4}$, which corresponds to a 3.5 $\sigma$ statistical significance.
    04/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: A fully reconstructed Bc --> Jpsi Pi signal is observed with the D0 detector at the Fermilab Tevatron p-pbar collider. Using 1.3 inverse femtobarns of integrated luminosity, the signal is extracted with a significance more than five standard deviations above background. The measured Bc meson mass is 6300 (+-) 14 (stat) (+-) 5 (sys) MeV/cc
    03/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present the first simultaneous measurement of the ratio of branching fractions, R=B(t->Wb)/B(t->Wq), with q being a d, s, or b quark, and the top quark pair production cross section sigma_ttbar in the lepton plus jets channel using 0.9 fb-1 of ppbar collision data at sqrt(s)=1.96 TeV collected with the D0 detector. We extract R and sigma_ttbar by analyzing samples of events with 0, 1 and >= 2 identified b jets. We measure R = 0.97 +0.09-0.08 (stat+syst) and sigma_ttbar = 8.18 +0.90-0.84 (stat+syst)} +/-0.50 (lumi) pb, in agreement with the standard model prediction.
    02/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We search for the production of single top quarks via flavor-changing neutral current couplings of a gluon to the top quark and a charm ($c$) or up ($u$) quark. We analyze 230 pb$^{-1}$ of lepton + jets data from $\ppbar$ collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters $\kappacLambda$ and $\kappauLambda$, where $\kappag$ define the strength of $tcg$ and $tug$ couplings, and $\Lambda$ defines the scale of new physics. The limits at 95% C.L. are: $\kappacLambda < 0.15 \rm TeV^{-1}$ and $\kappauLambda < 0.037 \rm TeV^{-1}$. Comment: 7 pages, 5 figures; updated (published) version of hep-ex/0702005
    01/2008;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present measurements of the process $p\bar{p} \to WZ+X \to \ell^{\prime} \nu_{\ell^{\prime}} \ell \bar{\ell}$ at $\sqrt{s}=1.96$ TeV, where $\ell$ and $\ell^{\prime}$ are electrons or muons. Using 1 fb$^{-1}$ of data from the D0 experiment, we observe 13 candidates with an expected background of $4.5\pm0.6$ events and measure a cross section $\sigma(WZ)=2.7^{+1.7}_{-1.3}$ pb. From the number of observed events and the $Z$ boson transverse momentum distribution, we limit the trilinear $WWZ$ gauge couplings to $-0.17 \le \lambda_Z \le 0.21$ $(\Delta \kappa_Z = 0)$ at the 95% C.L. for a form factor scale $\Lambda=2$ TeV. Further, assuming that $\Delta g^Z_1 = \Delta\kappa_Z$, we find $-0.12 \le \Delta\kappa_Z \le 0.29$ $(\lambda_Z=0)$ at the 95% C.L. These are the most restrictive limits on the $WWZ$ couplings available to date.
    10/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: Excited B mesons B_1 and B_2* are observed directly for the first time as two separate states in fully reconstructed decays to B+(*) pi-. The mass of B_1 is measured to be (5720.6 +- 2.4 +- 1.4) MeV/c^2 and the mass difference DeltaM between B_2* and B_1 is (26.2 +- 3.1 +- 0.9) MeV/c^2, giving the mass of the B_2* as (5746.8 +- 2.4 +- 1.7) MeV/c^2. The production rate for B_1 and B_2* mesons is determined to be a fraction (13.9 +- 1.9 +- 3.2)% of the production rate of the B+ meson.
    06/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: Long-lived, heavy particles are predicted in a number of models beyond the standard model of particle physics. We present the first direct search for such particles' decays, occurring up to 100 hours after their production and not synchronized with an accelerator bunch crossing. We apply the analysis to the gluino (~g), predicted in split supersymmetry, which after hadronization can become charged and lose enough momentum through ionization to come to rest in dense particle detectors. Approximately 410 pb^-1 of p-pbar collisions at 1.96 TeV collected with the D0 detector during Run II of the Fermilab Tevatron collider are analyzed in search of such ``stopped gluinos'' decaying into a gluon and a neutralino (~X_0), reconstructed as a jet and missing energy. No excess is observed above background, and limits are placed on the (gluino cross section) x (probability to stop) x BR(~g -> g ~X_0) as a function of the gluino and ~X_0 masses, for gluino lifetimes from 30 $\mu$s -- 100 hours.
    06/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We present a measurement of the top quark pair production cross section in ppbar collisions at sqrt(s)=1.96 TeV utilizing 425 pb-1 of data collected with the D0 detector at the Fermilab Tevatron Collider. We consider the final state of the top quark pair containing one high-pT electron or muon and at least four jets. We exploit specific kinematic features of ttbar events to extract the cross section. For a top quark mass of 175 GeV, we measure sigma_ttbar = 6.4 +1.3-1.2(stat} +/- 0.7(syst)+/- 0.4(lum) pb in good agreement with the standard model prediction. Comment: 24 pages, 17 figures, submitted to Phys. Rev. D
    05/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We describe a search for the standard model Higgs boson with a mass of 105 GeV/c^2 to 145 GeV/c^2 in data corresponding to an integrated luminosity of approximately 450 pb^{-1} collected with the D0 detector at the Fermilab Tevatron ppbar collider at a center-of-mass energy of 1.96 TeV. The Higgs boson is required to be produced in association with a Z boson, and the Z boson is required to decay to either electrons or muons with the Higgs boson decaying to a bbbar pair. The data are well described by the expected background, leading to 95% confidence level cross section upper limits sigma(\ppbar\to ZH)x B(H\to\bbbar) in the range of 3.1 pb to 4.4 pb.
    05/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We have performed the first direct measurement of the time integrated flavor untagged charge asymmetry in semileptonic Bs decays, Asl(unt), by comparing the decay rate of Bs --> mu+ Ds- nu X, where Ds- --> phi pi- and phi --> K+K-, with the charge-conjugate Bs-bar decay rate. This sample was selected from 1.3/fb of data collected by the D0 experiment in RunII of the Fermilab Tevatron collider. We obtain Asl(unt) = [1.23 +- 0.97(stat) +- 0.17(syst)]*10^-2. Assuming that Delta(M_s)/Gamma_s >> 1 and Delta(Gamma_s)/(2 Gamma_s) < 1, this result can be translated into a measurement on the CP-violating phase in Bs mixing: Delta(Gamma_s)/Delta(M_s)*tan(phi_s) = [2.45 +- 1.93(stat) +- 0.35(syst)]*10^-2.
    02/2007;
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    D0 Collaboration, V. M. Abazov
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    ABSTRACT: The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron ppbar collider. Using a 0.9 fb^-1 dataset, we apply a multivariate analysis to separate signal from background and measure sigma(ppbar->tb+X,tqb+X) = 4.9 +- 1.4 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the CKM matrix element that describes the Wtb coupling and find 0.68 < |Vtb| <= 1 at 95% C.L. within the standard model.
    01/2007;
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    D0 Collaboration, V. Abazov
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    ABSTRACT: We search for the technicolor process ppbar -> rho_T/omega_T ->W+pi_T in events containing one electron and two jets, in data corresponding to an integrated luminosity of 390 pb-1, recorded by the D0 experiment at the Fermilab Tevatron. Technicolor predicts that technipions, pi_T, decay dominantly into b-bbar, b-cbar, or bbar-c, depending on their charge. In these events b and c quarks are identified by their secondary decay vertices within jets. Two analysis methods based on topological variables are presented. Since no excess above the standard model prediction was found, the result is presented as an exclusion in the pi_T vs. rho_T mass plane for a given set of model parameters.
    01/2007;