Measurement of the t-channel single top quark production cross section

The D0 collaboration reports direct evidence for electroweak production of single top quarks through the t-channel exchange of a virtual W boson. This is the first analysis to isolate an individual single top quark production channel. We select events containing an isolated electron or muon, missing transverse energy, and two, three or four jets from 2.3 fb{sup -1} of p{bar p} collisions at the Fermilab Tevatron Collider. One or two of the jets are identified as containing a b hadron. We combine three multivariate techniques optimized for the t-channel process to measure the t- and s-channel cross sections simultaneously. We measure cross sections of 3.14{sub -0.80}{sup +0.94} pb for the t-channel and 1.05 {+-} 0.81 pb for the s-channel. The measured t-channel result is found to have a significance of 4.8 standard deviations and is consistent with the standard model prediction

Search for W'->tb resonances with left- and right-handed couplings to fermions

We present a search for the production of a heavy gauge boson, W{prime}, that decays to third-generation quarks, by the D0 Collaboration in p{bar p} collisions at {radical}s = 1.96 TeV. We set 95% confidence level upper limits on the production cross section times branching fraction. For the first time, we set limits for arbitrary combinations of left- and right-handed couplings of the W{prime} boson to fermions. For couplings with the same strength as the standard model W boson, we set the following limits for M(W{prime}) > m({nu}{sub R}): M(W{prime}) > 863 GeV for purely left-handed couplings, M(W{prime}) > 885 GeV for purely right-handed couplings, and M(W{prime}) > 916 GeV if both left- and right-handed couplings are present. The limit for right-handed couplings improves for M(W{prime}) < m({nu}{sub R}) to M(W{prime}) > 890 GeV

Combined Forward-Backward Asymmetry Measurements in Top-Antitop Quark Production at the Tevatron

The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of $\sqrt s =1.96$ TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is $A_{\mathrm{FB}}^{t\bar{t}} = 0.128 \pm 0.025$. The combined inclusive and differential asymmetries are consistent with recent standard model predictions

Measurement of Z/gamma*+jet+X angular distributions in p anti-p collisions at s**(1/2) = 1.96.TeV

We present the first measurements at a hadron collider of differential cross sections for Z/{gamma}* + jet + X production in {Delta}{phi}(Z, jet), |{Delta}y(Z, jet)| and |y{sub boost}(Z + jet)|. Vector boson production in association with jets is an excellent probe of QCD and constitutes the main background to many small cross section processes, such as associated Higgs production. These measurements are crucial tests of the predictions of perturbative QCD and current event generators, which have varied success in describing the data. Using these measurements as inputs in tuning event generators will increase the experimental sensitivity to rare signals

Search for WHWH associated production in ppˉp \bar{p} collisions at s=1.96 TeV\sqrt{s}=1.96\,{\rm TeV}

This report describes a search for associated production of W and Higgs bosons based on data corresponding to an integrated luminosity of L {approx} 5.3 fb{sup -1} collected with the D0 detector at the Fermilab Tevatron p{bar p} Collider. Events containing a W {yields} {ell}{nu} candidate (with {ell} corresponding to e or {mu}) are selected in association with two or three reconstructed jets. One or two of the jets are required to be consistent with having evolved from a b quark. A multivariate discriminant technique is used to improve the separation of signal and backgrounds. Expected and observed upper limits are obtained for the product of the WH production cross section and branching ratios and reported in terms of ratios relative to the prediction of the standard model as a function of the mass of the Higgs boson (M{sub H}). The observed and expected 95% C.L. upper limits obtained for an assumed M{sub H} = 115 GeV are, respectively, factors of 4.5 and 4.8 larger than the value predicted by the standard model

A measurement of the WZ and ZZ production cross sections using leptonic final states in 8.6 fb^{-1} of p\bar{p} collisions

We study the processes p{bar p} {yields} WZ {yields} {ell}{nu}{ell}{sup +}{ell}{sup -} and p{bar p} {yields} ZZ {yields} {ell}{sup +}{ell}{sup -}{nu}{bar {nu}}, where {ell} = e or {mu}. Using 8.6 fb{sup -1} of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron collider, we measure the WZ production cross section to be 4.50{sub -0.66}{sup +0.63} pb which is consistent with, but slightly above a prediction of the standard model. The ZZ cross section is measured to be 1.64 {+-} 0.46 pb, in agreement with a prediction of the standard model. Combination with an earlier analysis of the ZZ {yields} {ell}{sup +}{ell}{sup -}{ell}{sup +}{ell}{sup -} channel yields a ZZ cross section of 1.44{sub -0.34}{sup +0.35} pb

Model-independent measurement of t\boldsymbol{t}-channel single top quark production in ppˉ\boldsymbol{p\bar{p}} collisions at s=1.96\boldsymbol{\sqrt{s}=1.96} TeV

We present a model-independent measurement of t-channel electroweak production of single top quarks in p{bar p} collisions at {radical}s = 1.96 TeV. Using 5.4 fb{sup -1} of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider, and selecting events containing an isolated electron or muon, missing transverse energy and one or two jets originating from the fragmentation of b quarks, we measure a cross section {sigma}(p{bar p} {yields} tqb + X) = 2.90 {+-} 0.59 (stat + syst) pb for a top quark mass of 172.5 GeV. The probability of the background to fluctuate and produce a signal as large as the one observed is 1.6 x 10{sup -8}, corresponding to a significance of 5.5 standard deviations

An Improved determination of the width of the top quark

We present an improved determination of the total width of the top quark, {Lambda}{sub t}, using 5.4 fb{sup -1} of integrated luminosity collected by the D0 Collaboration at the Tevatron p{bar p} Collider. The total width {Lambda}{sub t} is extracted from the partial decay width {Lambda}(t {yields} Wb) and the branching fraction {Beta}(t {yields} Wb). {Lambda}(t {yields} Wb) is obtained from the t-channel single top quark production cross section and {Beta}(t {yields} Wb) is measured in t{bar t} events. For a top mass of 172.5 GeV, the resulting width is {Lambda}{sub t} = 2.00{sub -0.43}{sup +0.47} GeV. This translates to a top-quark lifetime of {tau}{sub t} = (3.29{sub -0.63}{sup +0.90}) x 10{sup -25} s. We also extract an improved direct limit on the CKM matrix element 0.81 < |V{sub tb}| {le} 1 at 95% C.L. and a limit of |V{sub tb'}| < 0.59 for a high mass fourth generation bottom quark assuming unitarity of the fourth generation quark mixing matrix

Search for WHWH associated production in 5.3 fb−1^{-1} of ppˉp\bar{p} collisions at the Fermilab Tevatron

We present a search for associated production of Higgs and W bosons in p{bar p} collisions at a center of mass energy of {radical}s = 1.96 TeV in 5.3 fb{sup -1} of integrated luminosity recorded by the D0 experiment. Multivariate analysis techniques are applied to events containing one lepton, an imbalance in transverse energy, and one or two b-tagged jets to discriminate a potential WH signal from standard model backgrounds. We observe good agreement between data and background, and set an upper limit of 4.5 (at 95% confidence level and for m{sub H} = 115 GeV) on the ratio of the WH cross section multiplied by the branching fraction of H {yields} b{bar b} to its standard model prediction. A limit of 4.8 is expected from simulation

Measurement of the production fraction times branching fraction f(b→Λb)⋅B(Λb→J/ψΛ)\boldsymbol{ f(b\to\Lambda_{b})\cdot \mathcal{B}(\Lambda_{b}\to J/\psi \Lambda)}

The {Lambda}{sub b}(udb) baryon is observed in the decay {Lambda}{sub b} {yields} J/{psi}{Lambda} using 6.1 fb{sup -1} of p{bar p} collisions collected with the D0 detector at {radical}s = 1.96 TeV. The production fraction multiplied by the branching fraction for this decay relative to that for the decay B{sup 0} {yields} J/{psi}K{sub s}{sup 0} is measured to be 0.345 {+-} 0.034 (stat.) {+-} 0.033 (syst.) {+-} 0.003 (PDG). Using the world average value of f(b {yields} B{sup 0}) {center_dot} {Beta}(B{sup 0} {yields} J/{psi}K{sub s}{sup 0}) = (1.74 {+-} 0.08) x 10{sup -5}, they obtain f(b {yields} {Lambda}{sub b}) {center_dot} {Beta}({Lambda}{sub b} {yields} J/{psi}{Lambda}) = (6.01 {+-} 0.60 (stat.) {+-} 0.58 (syst.) {+-} 0.28 (PDG)) x 10{sup -5}. This measurement represents an improvement in precision by about a factor of three with respect to the current world average