Wγ production and limits on anomalous WWγ couplings in pp collisions at sqrt[s] = 1.96 TeV

We measure the cross section and the difference in rapidities between photons and charged leptons for inclusive W(→lν) + γ production in eγ and μγ final states. Using data corresponding to an integrated luminosity of 4.2  fb(-1) collected with the D0 detector at the Fermilab Tevatron Collider, the measured cross section times branching fraction for the process pp → Wγ + X → lνγ + X and the distribution of the charge-signed photon-lepton rapidity difference are found to be in agreement with the standard model. These results provide the most stringent limits on anomalous WWγ couplings for data from hadron colliders: -0.4<Δκ(γ)<0.4 and -0.08<λ(γ)<0.07 at the 95% C.L.

Search for a fermiophobic and standard model Higgs boson in diphoton final states

We present a search for the standard model Higgs boson and a fermiophobic Higgs boson in the diphoton final states based on 8.2  fb(-1) of pp collisions at sqrt[s]=1.96  TeV collected with the D0 detector at the Fermilab Tevatron Collider. No excess of data above background predictions is observed and upper limits at the 95% C.L. on the cross section multiplied by the branching fraction are set which are the most restrictive to date. A fermiophobic Higgs boson with a mass below 112.9 GeV is excluded at the 95% C.L.

Search for neutral minimal supersymmetric standard model Higgs bosons decaying to tau pairs produced in association with b quarks in pp collisions at √s = 1.96 TeV

We report results from a search for neutral Higgs bosons produced in association with b quarks using data recorded by the D0 experiment at the Fermilab Tevatron Collider and corresponding to an integrated luminosity of 7.3 fb(-1). This production mode can be enhanced in several extensions of the standard model (SM) such as in its minimal supersymmetric extension (MSSM) at high tanβ. We search for Higgs bosons decaying to tau pairs with one tau decaying to a muon and neutrinos and the other to hadrons. The data are found to be consistent with SM expectations, and we set upper limits on the cross section times branching ratio in the Higgs boson mass range from 90 to 320 GeV/c(2). We interpret our result in the MSSM parameter space, excluding tanβ values down to 25 for Higgs boson masses below 170 GeV/c(2).

Precision measurement of the ratio B(t→Wb)/B(t→Wq) and extraction of V(tb)

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 tt final states. The measurement uses data from 5.4 fb(-1) of pp collisions collected with the D0 detector at the Fermilab Tevatron Collider. We measure R=0.90±0.04, and we extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V(tb)| as |V(tb)|=0.95±0.02, assuming unitarity of the 3×3 CKM matrix.

Search for a fourth generation t' Quark in p ̄p collisions at √s = 1.96 TeV

We present a search for pair production of a fourth generation t' quark and its antiparticle, followed by their decays to a W boson and a jet, based on an integrated luminosity of 5.3 fb(-1) of proton-antiproton collisions at √s = 1.96  TeV collected by the D0 Collaboration at the Fermilab Tevatron Collider. We set upper limits on the t' ̄t' production cross section that exclude at the 95% C.L. a t' quark that decays exclusively to W+jet with a mass below 285 GeV. We observe a small excess in the μ+jets channel which reduces the mass range excluded compared to the expected limit of 320 GeV in the absence of a signal.

Precise measurement of the top quark mass in the dilepton channel at D0

We measure the top quark mass (m(t)) in p ̄p collisions at a center of mass energy √s = 1.96 TeV using dilepton t ̄t→W(+)bW(-) ̄b→ℓ(+)ν(ℓ)bℓ(-) ̄ν(ℓ) ̄b events, where ℓ denotes an electron, a muon, or a tau that decays leptonically. The data correspond to an integrated luminosity of 5.4 fb(-1) collected with the D0 detector at the Fermilab Tevatron Collider. We obtain m(t)=174.0±1.8(stat)±2.4(syst) GeV, which is in agreement with the current world average m(t)=173.3±1.1 GeV. This is currently the most precise measurement of m(t) in the dilepton channel.

Indication of electron neutrino appearance from an accelerator-produced off-axis muon neutrino beam

The T2K experiment observes indications of ν(μ) → ν(e) appearance in data accumulated with 1.43×10(20) protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with |Δm(23)(2)| = 2.4×10(-3)  eV(2), sin(2)2θ(23) = 1 and sin(2)2θ(13) = 0, the expected number of such events is 1.5±0.3(syst). Under this hypothesis, the probability to observe six or more candidate events is 7×10(-3), equivalent to 2.5σ significance. At 90% C.L., the data are consistent with 0.03(0.04) < sin(2)2θ(13) < 0.28(0.34) for δ(CP) = 0 and a normal (inverted) hierarchy.

Measurement of spin correlation in tt production using a matrix element approach

We determine the fraction of tt events with spin correlation, assuming that the spin of the top quark is either correlated with the spin of the top antiquark as predicted by the standard model or is uncorrelated. For the first time we use a matrix-element-based approach to study tt spin correlation. We use tt → W+ b W- b → ℓ+ νbℓ- ν b final states produced in pp 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 D0 detector at the Fermilab Tevatron collider. The result agrees with the standard model prediction. We exclude the hypothesis that the spins of the tt are uncorrelated at the 97.7% C.L.

Search for Resonant WW and WZ Production in pp collisions at √s=1.96 TeV

We search for resonant WW or WZ production by 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 by using the sequential standard model W' boson and the Randall-Sundrum model graviton G as benchmarks. We exclude a sequential standard model W' boson in the mass range 180-690 GeV and a Randall-Sundrum graviton in the range 300-754 GeV at 95% C.L.

Bounds on an anomalous dijet resonance in W+jets production in pp collisions at √s = 1.96 TeV

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 √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 8×10(-6).

Search for the standard model Higgs boson in the H→WW→lνq'q decay channel

We present a search for the standard model Higgs boson (H) in pp collisions at sqrt(s)=1.96  TeV in events containing a charged lepton (ℓ), missing transverse energy, and at least two jets, using 5.4  fb(-1) of integrated luminosity recorded with the D0 detector at the Fermilab Tevatron Collider. This analysis is sensitive primarily to Higgs bosons produced through the fusion of two gluons or two electroweak bosons, with subsequent decay H→WW→ℓνq'q, where ℓ is an electron or muon. The search is also sensitive to contributions from other production channels, such as WH→ℓνbb. In the absence of a signal, we set limits at the 95% C.L. on the cross section for H production σ(pp→H+X) in these final states. For a mass of M(H)=160  GeV, the limit is a factor of 3.9 larger than the cross section in the standard model and consistent with an a priori expected sensitivity of 5.0.

Precise study of the Z/γ* boson transverse momentum distribution in pp collisions using a novel technique

Using 7.3 fb⁻¹ of pp collisions collected by the D0 detector at the Fermilab Tevatron, we measure the distribution of the variable φ(η)*, which probes the same physical effects as the Z/γ* 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 φ(η)* 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.

Search for single vectorlike quarks in pp ̄ collisions at √s=1.96 TeV

We present a search for hypothetical vectorlike quarks in pp ̅ collisions at √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 vectorlike quarks decaying to W+jet and Z+jet. These are the most stringent limits to date for electroweak single vectorlike quark production at hadron colliders.

Determination of the width of the top quark

We extract the total width of the top quark, Γ(t), from the partial decay width Γ(t → Wb) measured using the t-channel cross section for single top-quark production and from the branching fraction B(t → Wb) measured in tt events using up to 2.3  fb(-1) of integrated luminosity collected by the D0 Collaboration at the Tevatron pp Collider. The result is Γ(t) = 1.99(-0.55)(+0.69)  GeV, which translates to a top-quark lifetime of τ(t) = (3.3(-0.9)(+1.3)) × 10(-25)   s. Assuming a high mass fourth generation b' quark and unitarity of the four-generation quark-mixing matrix, we set the first upper limit on |V(tb')| < 0.63 at 95% C.L.

Search for ZH → l+ l- bb production in 4.2 fb(-1) of pp collisions at sqrt[s] =1 .96 TeV

We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb(-1) of pp collisions, collected with the D0 detector at the Fermilab Tevatron at sqrt[s] =1 .96 TeV. Selected events contain one reconstructed Z → e+ e- or Z → μ+ μ- candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M(H) = 115  GeV is a factor of 5.9 larger than the standard model prediction.

Search for diphoton events with large missing transverse energy in 6.3 fb(-1) of pp collisions at √s=1.96 TeV

We report a search for diphoton events with large missing transverse energy produced in pp collisions at √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 Λ is excluded for Λ<124  TeV. In a universal extra dimension model including gravitational decays, the compactification radius R(c) is excluded for R(c)(-1)<477  GeV.

Search for new fermions ("quirks") at the Fermilab Tevatron collider

We report results of a search for particles with anomalously high ionization in events with a high transverse energy jet and large missing transverse energy in 2.4 fb⁻¹ of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron pp collider. Production of such particles (quirks) is expected in scenarios with extra QCD-like SU(N) sectors, and this study is the first dedicated search for such signatures. We find no evidence of a signal and set a lower mass limit of 107, 119, and 133 GeV for the mass of a charged quirk with strong dynamics scale Λ in the range from 10 keV to 1 MeV and N=2, 3, and 5, respectively.

Search for events with leptonic jets and missing transverse energy in pp collisions at √s=1.96 TeV

We present the first search for pair production of isolated jets of charged leptons in association with a large imbalance in transverse energy in pp collisions using 5.8 fb⁻¹ of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider. No excess is observed above the standard model background, and the result is used to set upper limits on the production cross section of pairs of supersymmetric chargino and neutralino particles as a function of "dark-photon" mass, where the dark photon is produced in the decay of the lightest supersymmetric particle.

Search for sneutrino production in eμ final states in 5.3 fb-1 of pp collisions at square root s =1.96 TeV

We report the results of a search for R parity violating (RPV) interactions leading to the production of supersymmetric sneutrinos decaying into eμ final states using 5.3 fb-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider. Having observed no evidence for production of eμ resonances, we set direct bounds on the RPV couplings λ311' and λ312 as a function of sneutrino mass.

Evidence for an anomalous like-sign dimuon charge asymmetry

We measure the charge asymmetry A ≡ (N++ - N--)/(N++ + N--) of like-sign dimuon events in 6.1 fb-1 of pp collisions recorded with the D0 detector at a center-of-mass energy square root(s) = 1.96   TeV at the Fermilab Tevatron collider. From A we extract the like-sign dimuon charge asymmetry in semileptonic b-hadron decays: A(sl)(b) = -0.009 57 ± 0.002 51(stat) ± 0.001 46(sys). It differs by 3.2 standard deviations from the standard model prediction A(sl)(b)(SM) = (-2.3(-0.6)(+0.5)) × 10(-4), and provides first evidence of anomalous CP violation in the mixing of neutral B mesons.

Search for Randall-Sundrum gravitons in the dielectron and diphoton final states with 5.4 fb(-1) of data from pp collisions at square root(s) = 1.96 TeV

Using 5.4  fb(-1) of integrated luminosity from pp collisions at square root(s)=1.96  TeV collected by the D0 detector at the Fermilab Tevatron Collider, we search for decays of the lightest Kaluza-Klein mode of the graviton in the Randall-Sundrum model to ee and γγ. We set 95% C.L. lower limits on the mass of the lightest graviton between 560 and 1050 GeV for values of the coupling k/M(Pl) between 0.01 and 0.1.

Search for the associated production of a b quark and a neutral supersymmetric Higgs boson that decays into tau pairs

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 2.7 fb(-1) of pp collisions recorded by the D0 detector at square root(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.

Search for the standard model higgs Boson in the ZH-->nunubb channel in 5.2 fb{-1} of pp collisions at sqrt[s]=1.96 TeV

A search is performed for the standard model Higgs boson in 5.2 fb{-1} of pp collisions at sqrt[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-->nunubb production. The search is also sensitive to the WH-->lnubb 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.

Search for a resonance decaying into WZ boson pairs in pp 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, mu). 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.