We report the results of a search for a heavy neutral gauge boson Z super(') decaying into the dielectron final state using data corresponding to an integrated luminosity of 5.4 fb super(-1) ...collected by the D0 experiment at the Fermilab Tevatron Collider. No significant excess above the standard model prediction is observed in the dielectron invariant-mass spectrum. We set inline image depending on the dielectron invariant mass. These cross section limits are used to determine lower mass limits for Z super(') bosons in a variety of models. For the sequential standard model Z super(') boson a lower mass limit of 1023 GeV is obtained.
We present a new measurement of the WZ-->l nu ll (l=e, mu ) cross section and limits on anomalous triple gauge couplings. Using 4.1 fb super(-1) of integrated luminosity of inline image collisions at ...inline image, we observe 34 WZ candidate events with an estimated background of 6.0+/-0.4 events. We measure the WZ production cross section to be inline image, in good agreement with the standard model prediction. We find no evidence for anomalous WWZ couplings and set 95% C.L. limits on the coupling parameters, -0.077< lambda sub(Z)<0.093 and -0.029< Delta Kappa sub(Z)<0.080 in the HISZ parameterization for a Lambda =2 TeV form factor scale. These are the best limits to date obtained from the direct measurement of the WWZ vertex.
We present a search for pair production of doubly-charged Higgs bosons in the processes qqbar to H++H-- decaying through H++/-- to tau tau, muon tau, muon muon. The search is performed in ppbar ...collisions at a center-of-mass energy of sqrt{s}=1.96 TeV using an integrated luminosity of 7.0 fb-1 collected by the D0 experiment at the Fermilab Tevatron Collider. The results are used to set 95% C.L. limits on the pair production cross section of doubly-charged Higgs bosons and on their mass for different H++/-- branching fractions. Models predicting different H++/-- decays are investigated. Assuming BR(H++ -> tau+ tau+)=1 yields an observed (expected) lower limit on the mass of a left-handed H++/-- boson of 128 (116) GeV and assuming BR(H++ -> mu+ tau+)=1 the corresponding limits are 144 (149) GeV. In a model with BR(H++ -> tau+ tau+)=BR(H++ -> mu+ tau+)=BR(H++ -> mu+ mu+)=1/3, we obtain M(H++(L) > 130 (138) GeV.
We present a study of the dijet invariant mass spectrum in events with two jets produced in association with a $W$ boson in data corresponding to an integrated luminosity of 4.3 fb$^{-1}$ collected ...with the D0 detector at $\sqrt{s} =1.96$ TeV. We find no evidence for anomalous resonant dijet production and derive upper limits on the production cross section of an anomalous dijet resonance recently reported by the CDF Collaboration, investigating the range of dijet invariant mass from 110 to 170 GeV/$c^2$. The probability of the D0 data being consistent with the presence of a dijet resonance with 4 pb production cross section at 145 GeV/$c^2$ is $8\times 10^{-6}$.
We determine the fraction of tt̅ events with spin correlation, assuming that the spin of the top quark is either correlated with the spin of the anti-top quark as predicted by the standard model or ...is uncorrelated. For the first time we use a matrix-element-based approach to study ttbar spin correlation. We use {ttbar -> W+bW-bbar ->l+nubl-nub} final states produced in ppbar collisions at a center of mass energy sqrt(s)=1.96 TeV, where l denotes an electron or a muon. The data correspond to an integrated luminosity of 5.4 fb-1 and were collected with the dzero detector at the Fermilab Tevatron collider. The result agrees with the standard model prediction. We exclude the hypothesis that the spins of the ttbar are uncorrelated at the 97.7% C.L.
Using 7.3 pb-1 of ppbar collisions collected by the D0 detector at the Fermilab Tevatron, we measure the distribution of the variable \phistar, which probes the same physical effects as the Z/gamma* ...boson transverse momentum, but is less susceptible to the effects of experimental resolution and efficiency. A QCD prediction is found to describe the general features of the \phistar distribution, but is unable to describe its detailed shape or dependence on boson rapidity. A prediction that includes a broadening of transverse momentum for small values of the parton momentum fraction is strongly disfavored.
We search for resonant WW or WZ production using up to 5.4 fb-1 of integrated luminosity collected by the D0 experiment in Run II of the Fermilab Tevatron Collider. The data are consistent with the ...standard model background expectation, and we set limits on a resonance mass using the sequential standard model (SSM) W' boson and the Randall-Sundrum model graviton G as benchmarks. We exclude an SSM W' boson in the mass range 180 - 690 GeV and a Randall-Sundrum graviton in the range 300 - 754 GeV at 95% CL.
We present a search for hypothetical vector-like quarks in ppbar collisions at sqrt(s) = 1.96 TeV. The data were collected by the D0 detector at the Fermilab Tevatron Collider and correspond to an ...integrated luminosity of 5.4 fb^(-1). We select events with a final state composed of a W or Z boson and a jet consistent with a heavy object decay. We observe no significant excess in comparison to the background prediction and set limits on production cross sections for vector-like quarks decaying to W+jet and Z+jet. Assuming a vector-like quark to standard model quark coupling parameter $\tilde{\ kappa}_{qQ}$ of unity, we exclude vector-like quarks with mass below 693 GeV for decays to W+jet and mass below 449 GeV for decays to Z+jet. These are the most stringent mass limits for electroweak single vector-like quark production at hadron colliders.
We present a measurement of the ratio of top quark branching fractions R = B(t -> Wb)/B(t -> Wq), where q can be a d, s or b quark, in the lepton+jets and dilepton ttbar final states. The measurement ...uses data from 5.4 fb-1 of ppbar collisions collected with the D0 detector at the Fermilab Tevatron Collider. We measure R = 0.90 +/- 0.04, and we extract the CKM matrix element |Vtb| as |Vtb| = 0.95 +/- 0.02, assuming unitarity of the 3x3 CKM matrix.
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