arXiv subjects

V. Abazov

Publications and source records attributed to V. Abazov.

At least 109 records · Page 6Linked to original sources

arXiv2008

Search for production of single top quarks via $tcg$ and $tug$ flavor-changing neutral current couplings

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 $Λ$ defines the scale of new physics. The limits at 95% C.L. are: $\kappacLambda < 0.15 \rm TeV^{-1}$ and $\kappauLambda < 0.037 \rm TeV^{-1}$.

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arXiv2008

Simultaneous measurement of the ratio B(t->Wb)/B(t->Wq) and the top quark pair production cross section with the D0 detector at sqrt(s)=1.96 TeV

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.

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arXiv2008

Search for excited electrons in ppbar collisions at sqrt(s) = 1.96 TeV

We present the results of a search for the production of an excited state of the electron, e*, in proton-antiproton collisions at sqrt(s) = 1.96 TeV. The data were collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 1 fb^-1. We search for e* in the process ppbar -> e* e, with the e* subsequently decaying to an electron plus photon. No excess above the standard model background is observed. Interpreting our data in the context of a model that describes e* production by four-fermion contact interactions and e* decay via electroweak processes, we set 95% C.L. upper limits on the production cross section ranging from 8.9 fb to 27 fb, depending on the mass of the excited electron. Choosing the scale for contact interactions to be Lambda = 1 TeV, excited electron masses below 756 GeV are excluded at the 95% C.L.

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arXiv2008

Search for the lightest scalar top quark in events with two leptons in p-pbar collisions at sqrt(s) = 1.96 TeV

Data collected by the D0 detector at a p-pbar center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider have been used to search for pair production of the lightest supersymmetric partner of the top quark decaying into $b \ell \tildeν$. The search is performed in the $\ell\ell' = eμ$ and $μμ$ final states. No evidence for this process has been found in data samples of approximately 400 pb^-1. The domain in the [$M(\tilde{t}_1),M(\tildeν)$] plane excluded at the 95% C.L. is substantially extended by this search.

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arXiv2007

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

We present a study of eegamma and mumugamma events using 1109 (1009) pb-1 of data in the electron (muon) channel, respectively. These data were collected with the D0 detector at the Fermilab Tevatron ppbar Collider at sqrt(s) = 1.96 TeV. Having observed 453 (515) candidates in the eegamma (mumugamma) final state, we measure the Zgamma production cross section for a photon with transverse energy ET > 7 GeV, separation between the photon and leptons Delta R(lgamma) > 0.7, and invariant mass of the di-lepton pair M(ll) > 30 GeV/c^2, to be 4.96 +/- 0.30(stat. + syst.) +/- 0.30(lumi.) pb, in agreement with the standard model prediction of 4.74 +/- 0.22 pb. This is the most precise Zgamma cross section measurement at a hadron collider. We set limits on anomalous trilinear Zgammagamma and ZZgamma gauge boson couplings of -0.085 < h(30)^(gamma) < 0.084, -0.0053 < h(40)^(gamma) < 0.0054 and -0.083 < h(30)^(Z) < 0.082, -0.0053 < h(40)^(Z) < 0.0054 at the 95% C.L. for the form-factor scale Lambda = 1.2 TeV.

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arXiv2007

Search for ZZ and Zγ^* production in p-barp collisions at sqrt(s) = 1.96 TeV and limits on anomalous ZZZ and ZZγ^* couplings

We present a study of four muon, four electron, and two muon two electron events using 1 fb^(-1) of data collected with the D0 detector at the Fermilab Tevatron p-barp Collider at sqrt(s) = 1.96 TeV. Requiring the lepton pair masses to be greater than 30 GeV, we observe one event, consistent with the expected background of 0.13 +- 0.03 events and with the predicted standard model ZZ and Zγ^* production of 1.71 +-0.15 events. We set an upper limit on the ZZ and Zγ^* cross section of 4.4 pb at the 95% C.L. We also derive limits on anomalous neutral trilinear ZZZ and ZZγ^* gauge couplings. The one-parameter 95%$ C.L. coupling limits with a form factor scale Lambda = 1.2 TeV are -0.28 < f_(40)^Z < 0.28, -0.31 < f_(50)^Z < 0.29, -0.26 < f_(40)^γ< 0.26, and -0.30 < f_(5 0)^γ< 0.28.

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arXiv2007

Search for Randall-Sundrum gravitons with 1 fb-1 of data from ppbar collisions at sqrt(s)=1.96 TeV

Using 1 fb-1 of data from ppbar collisions at sqrt(s)=1.96 TeV at the Fermilab Tevatron collider collected by the D0 detector, we search for decays of Kaluza-Klein excitations of the graviton in the Randall-Sundrum model of extra dimensions to e+e- and diphotons. We set 95% confidence level upper limits on the production cross section times branching fraction which translate into lower limits on the mass of the lightest excitation between 300 and 900 GeV for values of the coupling k/M(Planck) between 0.01 and 0.1.

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arXiv2007

Search for W' bosons decaying to an electron and a neutrino with the D0 detector

This Letter describes the search for a new heavy charged gauge boson W' decaying into an electron and a neutrino. The data were collected with the D0 detector at the Fermilab Tevatron proton-antiproton Collider at a center-of-mass energy of 1.96 TeV, and correspond to an integrated luminosity of about 1 inverse femtobarn. Lacking any significant excess in the data in comparison with known processes, an upper limit is set on the production cross section times branching fraction, and a W' boson with mass below 1.00 TeV can be excluded at the 95% C.L., assuming standard-model-like couplings to fermions. This result significantly improves upon previous limits, and is the most stringent to date.

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arXiv2007

Measurement of the p\bar{p}\to WZ + X cross section at \sqrt{s}=1.96 TeV and limits on WWZ trilinear gauge couplings

We present measurements of the process $p\bar{p} \to WZ+X \to \ell^{\prime} ν_{\ell^{\prime}} \ell \bar{\ell}$ 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 $σ(WZ)=2.7^{+1.7}_{-1.3}$ pb. From the number of observed events and the $Z$ boson transverse momentum distribution, we limit the trilinear $WWZ$ gauge couplings to $-0.17 \le λ_Z \le 0.21$ $(Δκ_Z = 0)$ at the 95% C.L. for a form factor scale $Λ=2$ TeV. Further, assuming that $Δg^Z_1 = Δκ_Z$, we find $-0.12 \le Δκ_Z \le 0.29$ $(λ_Z=0)$ at the 95% C.L. These are the most restrictive limits on the $WWZ$ couplings available to date.

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arXiv2007

Direct observation of the strange b baryon Xi_b^{-}

We report the first direct observation of the strange b baryon Xi_b^{-}. We reconstruct the decay Xi_b^{-} to J/psi Xi^{-}, with J/psi to dimuons and Xi^{-} to Lambda pion, in ppbar collisions at sqrt(s) = 1.96 TeV. Using 1.3 fb^{-1} of data collected by the D0 detector, we observe 15.2 +/- 4.4(stat.)+ 1.9/-0.4(syst.) Xi_b^{-} candidates at a mass of 5.774 +/- 0.011(stat.) +/- 0.015 (syst.) GeV. The significance of the observed signal is 5.5 sigmas, equivalent to a probability of 3.3 X 10^{-8} of it arising from a background fluctuation. Normalizing to the decay Lambda_b to J/psi Lambda, we measure the relative rate to be 0.28 +/- 0.09(stat.)+ 0.09/-0.08 (syst.).

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arXiv2007

Measurement of the Lambda_b lifetime in the exclusive decay Lambda_b to J/psi Lambda

We have measured the Lambda_b lifetime using the exclusive decay Lambda_b to J/psi Lambda, based on 1.2 fb-1 of data collected with the D0 detector during 2002-2006. From 171 reconstructed Lambda_b decays, where the J/psi and Lambda are identified via the decays J/psi to mu+ mu- and Lambda to p pi, we measured the Lambda_b lifetime to be tau(Lambda_b) = 1.218(+0.130,-0.115)(stat) +/- 0.042(syst)ps. We also measured the B0 lifetime in the decay B0 to J/psi(mu+ mu-)K0_S(pi+ pi-) to be tau(B0) = 1.501(+0.078,-0.074)(stat) +/ -0.050(syst)ps, yielding a lifetime ratio of tau(Lambda_b)/tau(B0) = 0.811(+0.096,-0.087)(stat) +/ -0.034(syst). These measurements are consistent with the current world averages and support the shorter lifetime of the Lambda_b with respect to B mesons, in contrast to another recent measurement of significant precision.

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arXiv2007

Search for stopped gluinos from p-pbar collisions at sqrt(s)=1.96 TeV

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 $μ$s -- 100 hours.

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arXiv2007

Search for flavor-changing-neutral-current D meson decays

We study the flavor-changing-neutral-current process c to u mu+ mu- using 1.3 fb^-1 of p p bar collisions at sqrt(s) = 1.96 TeV recorded by the D0 detector operating at the Fermilab Tevatron Collider. We see clear indications of the Ds+ and D+ to phi pi+ to mu+ mu- pi+ final states with significance greater than four standard deviations above background for the D+ state. We search for the continuum decay of D+ to pi+mu+mu- in the dimuon invariant mass spectrum away from the phi resonance. We see no evidence of signal above background and set a limit of B(D+ to pi+mu+mu-) < 3.9 x 10^-6 at the 90% C.L. This limit places the most stringent constraint on new phenomena in the c to u mu+ mu- transition.

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arXiv2007

Search for Bs --> mu+ mu- at D0

We report results from a search for the decay Bs --> mu+ mu- using 1.3/fb of pbar p collisions at sqrt(s)= 1.96 TeV collected by the D0 experiment at the Fermilab Tevatron Collider. We find two candidate events, consistent with the expected background of 1.24 +/- 0.99, and set an upper limit on the branching fraction of B(Bs --> mu+ mu-) < 1.2e-7 at the 95% C.L.

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arXiv2007

Measurement of the ttbar production cross section in ppbar collisions using dilepton events

We present a measurement of the ttbar pair production cross section in ppbar collisions at sqrt(s) = 1.96 TeV utilizing approximately 425 pb-1 of data collected with the D0 detector. We consider decay channels containing two high pT charged leptons (either e or μ) from leptonic decays of both top-daughter W bosons. These were gathered using four sets of selection criteria, three of which required that a pair of fully identified leptons (i.e., eμ, ee, or μμ) be found. The fourth approach imposed less restrictive criteria on one of the lepton candidates and required that at least one hadronic jet in each event be tagged as containing a b quark. For a top quark mass of 175 GeV, the measured cross section is 7.4 +/-1.4(stat} +/- 1.0(syst) pb.

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arXiv2007

Search for particles decaying into a Z boson and a photon in p\bar{p} collisions at \sqrt{s} = 1.96 TeV

We present the results of the first search for a new particle X produced in $p\bar{p}$ collisions at \sqrt{s}=1.96 TeV and subsequently decaying to Zγ. The search uses 0.3 fb$^{-1}$ of data collected with the DØdetector at the Fermilab Tevatron Collider. We set limits on the production cross section times the branching fraction $σ(p \bar{p} \to X) \times B(X\to Zγ)$ that range from 0.4 -- 3.5 pb at the 95% C.L. for $X$ with invariant masses between 100 and 1000 GeV/$c^2$ over a wide range of $X$ decay widths.

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arXiv2007

Measurement of the Lambda_b lifetime using semileptonic decays

We report a measurement of the Lambda_b lifetime using a sample corresponding to 1.3 fb$^{-1}$ of data collected by the D0 experiment in 2002--2006 during Run II of the Fermilab Tevatron collider. The Lambda_b baryon is reconstructed via the decay Lambda_b -> mu nu Lambda_c X. Using $4437 \pm 329$ signal candidates, we measure the Lambda_b lifetime to be $τ(Lambda_b)$ = 1.290^{+0.119}_{-0.110}(stat) ^{+0.087}_{-0.091} (syst) ps, which is among the most precise measurements in semileptonic Lambda_b decays. This result is in good agreement with the world average value.

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arXiv2007

Observation and Properties of L=1 B_1 and B_2* Mesons

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.

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