Search for the Standard Model Higgs Boson in the \u3ci\u3eZH\u3c/i\u3e → νν\u3ci\u3ebb\u3c/i\u3e Channel in 5.2 fb\u3csup\u3e-1\u3c/sup\u3e of \u3ci\u3epp\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

A search is performed for the standard model Higgs boson in 5.2 fb-1 of pp collisions at √s = 1.96 TeV, collected with the D0 detector at the Fermilab Tevatron Collider. The final state considered is a pair of b jets and large missing transverse energy, as expected from pp → ZH → ννbb production. The search is also sensitive to the WH → ℓνbb channel when the charged lepton is not identified. For a Higgs boson mass of 115 GeV, a limit is set at the 95% C.L. on the cross section multiplied by branching fraction for [pp → (Z/W)H](H → bb) that is a factor of 3.7 larger than the standard model value, consistent with the factor of 4.6 expected

Measurement of the \u3ci\u3ett\u3c/i\u3e Production Cross Section in \u3ci\u3epp\u3c/i\u3e Collisions at √\u3ci\u3es\u3c/i\u3e = 1.96 TeV

We measure the tt production cross section in pp collisions at √s = 1.96 TeV in the lepton + jets channel. Two complementary methods discriminate between signal and background: b tagging and a kinematic likelihood discriminant. Based on 0.9 fb-1 of data collected by the D0 detector at the Fermilab Tevatron Collider, we measure σtt = 7.62 ± 0.85 pb, assuming the current world average mt = 172.6 GeV. We compare our cross section measurement with theory predictions to determine a value for the top-quark mass of 170 ± 7 GeV

Search for a Resonance Decaying into \u3ci\u3eWZ\u3c/i\u3e Boson Pairs in \u3ci\u3epp\u3c/i\u3e Collisions

We present the first search for an electrically charged resonance W’ decaying to a WZ boson pair using 4.1 fb-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron pp collider. The WZ pairs are reconstructed through their decays into three charged leptons (ℓ = e, μ). A total of 9 data events is observed in good agreement with the background prediction. We set 95% C.L. limits on the W’WZ coupling and on the W’ production cross section multiplied by the branching fractions. We also exclude W’ masses between 188 and 520 GeV within a simple extension of the standard model and set the most restrictive limits to date on low-scale technicolor models

Evidence for production of single top quarks

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron pp collider. The standard model predicts that the electroweak interaction can produce a top quark together with an anti-bottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a 0.9 fb-1 data set that have an electron or muon and missing transverse energy from the decay of a W boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W + jets and tt events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of σ(pp Y→ tb + X, tqb + X) = 4.7 ± 1:3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi- Maskawa quark mixing matrix element that describes the Wtb coupling and find |VtbƒL1 | = 1.31-0.21+0.25 , where ƒL1 is a generic vector coupling. This model-independent measurement translates into 0.68 \u3c |Vtb| ≤ 1 at the 95% C.L. in the standard model

Observation of Single Top-Quark Production

We report observation of the electroweak production of single top quarks in pp collisions at √s = 1.96 TeV based on 2.3 fb-1 of data collected by the D0 detector at the Fermilab Tevatron Collider. Using events containing an isolated electron or muon and missing transverse energy, together with jets originating from the fragmentation of b quarks, we measure a cross section of σ(pp → tb + X, tqb + X) = 3.94 ± 0.88 pb. The probability to measure a cross section at this value or higher in the absence of signal is 2.5 x 10-7, corresponding to a 5.0 standard deviation significance for the observation

Evidence for Production of Single Top Quarks and First Direct Measurement of |\u3ci\u3eV\u3c/i\u3e\u3csub\u3e\u3ci\u3etb\u3c/i\u3e\u3c/sub\u3e|

The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron pp̅ collider. Using a 0.9 fb-1 dataset, we apply a multivariate analysis to separate signal from background and measure σ (pp̅ → 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 a signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the Cabibbo- Kobayashi-Maskawa matrix element that describes the Wtb coupling and find 0.68 \u3c |V tb| ≤ 1 at 95% C.L. within the standard model

Direct Measurement of the Mass Difference between Top and Antitop Quarks

We present a measurement of the mass difference between t and t quarks in lepton + jets final states of tt events in 1 fb-1 of data collected with the D0 detector from Fermilab Tevatron Collider pp collisions at √s = 1.96 TeV. The measured mass difference of 3.8 ± 3.7 GeV is consistent with the equality of t and t masses. This is the first direct measurement of a mass difference between a quark and its antiquark partner

Measurement of the differential cross section for the production of an isolated photon with associated jet in \u3ci\u3epp\u3c/i\u3e collisions at √s = 1.96 TeV

The process pp → γ + jet + X is studied using 1.0 fb−1 of data collected by the D0 detector at the Fermilab Tevatron pp collider at a center-of-mass energy √s = 1.96 TeV. Photons are reconstructed in the central rapidity region |yγ | \u3c 1.0 with transverse momenta in the range 30 \u3c pγT \u3c 400 GeV while jets are reconstructed in either the central |yjet| \u3c 0.8 or forward 1.5 \u3c |yjet| \u3c 2.5 rapidity intervals with pjetT \u3e 15 GeV. The differential cross section d3σ/dpγT dyγ dyjet is measured as a function of pγT in four regions, differing by the relative orientations of the photon and the jet in rapidity. Ratios between the differential cross sections in each region are also presented. Next-to-leading order QCD predictions using different parameterizations of parton distribution functions and theoretical scale choices are compared to the data. The predictions do not simultaneously describe the measured normalization and pγT dependence of the cross section in the four measured regions

Search for the lightest scalar top quark in events with two leptons in \u3ci\u3epp\u3c/i\u3e collisions at √s = 1.96 TeV

Data collected by the D0 detector at a pp 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 bℓν. The search is performed in the ℓℓ′ = 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(t1),M(ν)] plane excluded at the 95% C.L. is substantially extended by this search

Search for Techniparticles in ℯ + jets Events at D0

Fermilab Tevatron. Technicolor predicts that technipions π T decay dominantly into bb̅, bc̅, or b̅c, 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 πT vs ρ T mass plane for a given set of model parameters