Search

We present a simultaneous measurement of the forward-backward asymmetry and the top-quark polarization in t... production in dilepton final states using 9.7 fb- of proton-antiproton collisions at √s = 1.96 TeV with the DO detector. To reconstruct the distributions of kinematic observables we employ a matrix element technique that calculates the likelihood of the possible t... kinematic configurations. After accounting for the presence of background events and for calibration effects, we obtain a forward-backward asymmetry of At... = (15.0 ± 6.4(stat) ±4.9(syst))% and a top-quark polarization times spin analyzing power in the beam basis of kP = (7.2 ± 10.5(stat) ± 4.2(syst))%, with a correlation of -56% between the measurements. If we constrain the forward-backward asymmetry to its expected standard model value, we obtain a measurement of the top polarization of ΚP = (11.3 ± 9.1(stat) ± 1.9(syst))%. If we constrain the top polarization to its expected standard model value, we measure a forward-backward asymmetry of At... = (17.5 ± 5.6(stat) ±3.1 (syst))%. A combination with the DO A" measurement in the lepton + jets final state yields an asymmetry of At... = (11.8 ± 2.5(stat) ± 1.3(syst))%. Within their respective uncertainties, all these results are consistent with the standard model expectations. [ABSTRACT FROM AUTHOR]

We measure the forward-backward asymmetry in the production of Λ0b and &Λmacr;0b baryons as a function of rapidity in pp collisions at s√ = 1.96  TeV using 10.4  fb−1 of data collected with the D0 detector at the Fermilab Tevatron collider. The asymmetry is determined by the preference of Λ0b or &Λmacr;0b particles to be produced in the direction of the beam protons or antiprotons, respectively. The measured asymmetry integrated over rapidity y in the range 0.1 < |y| < 2.0 is A = 0.04 ± 0.07(stat) ± 0.02(syst). [ABSTRACT FROM AUTHOR]

We present a measurement of the distribution of the variable ϕ*η for muon pairs with masses between 30 and 500 GeV, using the complete run II data set collected by the D0 detector at the Fermilab Tevatron proton-antiproton collider. This corresponds to an integrated luminosity of 10.4 fb−1 at s√=1.96  TeV. The data are corrected for detector effects and presented in bins of dimuon rapidity and mass. The variable ϕ*η probes the same physical effects as the Z/γ* boson transverse momentum, but is less susceptible to the effects of experimental resolution and efficiency. These are the first measurements at any collider of the ϕ*η distributions for dilepton masses away from the Z → ℓ+ℓ− boson mass peak. The data are compared to QCD predictions based on the resummation of multiple soft gluons. [ABSTRACT FROM AUTHOR]

We measure the ratio of cross sections, σ(pp → Z + 2b jets)/σ(pp → Z + 2 jets), for associated production of a Z boson with at least two jets with transverse momentum pTjet > 20 GeV and pseudorapidity |ηjet| < 2.5. This measurement uses data corresponding to an integrated luminosity of 9.7 fb-1 collected by the D0 experiment in Run II of Fermilab's Tevatron pp Collider at a center-of-mass energy of 1.96 TeV. The measured integrated ratio of 0.0236±0.0032(stat) ± 0.0035(syst) is in agreement with predictions from next-to-leading-order perturbative QCD and the Monte Carlo event generators pythia and alpgen. [ABSTRACT FROM AUTHOR]

We present a measurement of the electron charge asymmetry in pp → W + X → eν + X events at a center-of-mass energy of 1.96 TeV, using data corresponding to 9.7 fb-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider. The asymmetry is measured as a function of the electron pseudorapidity and is presented in five kinematic bins based on the electron transverse energy and the missing transverse energy in the event. The measured asymmetry is compared with next-to-leading-order predictions in perturbative quantum chromodynamics and provides accurate information for the determination of parton distribution functions of the proton. This is the most precise lepton charge asymmetry measurement to date. [ABSTRACT FROM AUTHOR]

We measure the direct CP-violating parameter ACP for the decay of the charged charm meson, D+ → K- π+ π- (and charge conjugate), using the full 10.4 fb-1 sample of pp collisions at √s = 1.96 TeV collected by the DO detector at the Fermilab Tevatron collider. We extract the raw reconstructed charge asymmetry by fitting the invariant mass distributions for the sum and difference of charge-specific samples. This quantity is then corrected for detector-related asymmetries using data-driven methods and for possible physics asymmetries (from B → D processes) using input from Monte Carlo simulation. We measure ACP = [-0.16 ± 0.15(stat) ± 0.09(syst)]%, which is consistent with zero, as expected from the standard model prediction of CP conservation, and is the most precise measurement of this quantity to date. [ABSTRACT FROM AUTHOR]

We present the observation of doubly produced J/Ψ mesons with the DO detector at Fermilab in pp collisions at √s = 1.96 TeV. The production cross section for both singly and doubly produced J/Ψ mesons is measured using a sample with an integrated luminosity of 8.1 fb-1. For the first time, the double J/Ψ production cross section is separated into contributions due to single and double parton scatterings. Using these measurements, we determine the effective cross section (jeff, a parameter characterizing an effective spatial area of the parton-parton interactions and related to the parton spatial density inside the nucleón. [ABSTRACT FROM AUTHOR]

The production of top quark-antiquark pair events in pp collisions at √s = 1.96 TeV is studied as a function of the transverse momentum and absolute value of the rapidity of the top quarks as well as of the invariant mass of the tt pair. We select events containing an isolated lepton, a large imbalance in transverse momentum, and four or more jets with at least one jet identified as originating from a b quark. The data sample corresponds to 9.7 fb-1 of integrated luminosity recorded with the DO detector during Run II of the Fermilab Tevatron Collider. Observed differential cross sections are consistent with standard model predictions. [ABSTRACT FROM AUTHOR]

We present a measurement of the forward-backward asymmetry in top quark-antiquark production using the full Tevatron Run II data set collected by the DO experiment at Fermilab. The measurement is performed in lepton + jets final states using a new kinematic fitting algorithm for events with four or more jets and a new partial reconstruction algorithm for events with only three jets. Corrected for detector acceptance and resolution effects, the asymmetry is evaluated to be AFB = (10.6 ±3.0)%. Results are consistent with the standard model predictions which range from 5.0% to 8.8%. We also present the dependence of the asymmetry on the invariant mass of the top quark-antiquark system and the difference in rapidities of the top quark and antiquark. [ABSTRACT FROM AUTHOR]

We present measurements of the forward-backward asymmetry in the angular distribution of leptons from decays of top quarks and antiquarks produced in proton-antiproton collisions. We consider the final state containing a lepton and at least three jets. The entire sample of data collected by the DO experiment during Run E of the Fermilab Tevatron Collider, corresponding to 9.7 fb-1 of integrated luminosity, is used. The asymmetry measured for reconstructed leptons is ... . When corrected for efficiency and resolution effects within the lepton rapidity coverage of |yl| < 1.5, the asymmetry is found to be ... . We examine the dependence of AlFB on the transverse momentum and rapidity of the lepton. Combination with the asymmetry measured in the final states containing two leptons yields ... . The results are in agreement with predictions from the next-to-leading-order QCD generator MC@NLO, which predicts an asymmetry of .... [ABSTRACT FROM AUTHOR]

We present a measurement of the electric charge of top quarks using tt events produced in pp collisions at the Tevatron. The analysis is based on fully reconstructed ft pairs in lepton + jets final states. Using data corresponding to 5.3 fb-1 of integrated luminosity, we exclude the hypothesis that the top quark has a charge of Q = -4 /3 e at a significance greater than 5 standard deviations. We also place an upper limit of 0.46 on the fraction of such quarks that can be present in an admixture with the standard model top quarks (Q = +2/3e) at a 95% confidence level. [ABSTRACT FROM AUTHOR]

We determine the fraction of events with double parton (DP) scattering in a single pp collision at √s = 1.96 TeV in samples of γ + 3 jet and γ + b/c jet + 2 jet events collected with the D0 detector and corresponding to an integrated luminosity of about 8.7 fb-1. The DP fractions and effective cross sections (σeff) are measured for both event samples using the same kinematic selections. The measured DP fractions range from 0.21 to 0.17, with effective cross sections in the γ + 3 jet and γ + b/cjet + 2 jet samples of σeffincl = 12.7±0.2(stat)±1.3(syst) mb and σeffHF = 14.6±0.6(stat)±3.2(syst) mb, respectively. [ABSTRACT FROM AUTHOR]

We present a measurement of Z boson pair production in pp̄ collisions at 1.96 TeV with 9.6 to 9.8 fb-1 of D0 data. We examine the final states eeee, eeμμ, and μμμμ. Based on selected data, the measured cross section in the mass region M(Z/γ*) > 30 GeV is σ(pp̄ → Z/γ* Z/γ*) = 1.26-0.36+0.44(stat)-0.15+0.17(syst) ± 0.08(lumi) pb; after correcting for the expected ratio of σ(pp̄ → Z/γ* Z/γ*) to σ(pp̄ → ZZ), we derive a cross section for pp̄ → ZZ production of 1.05-0.30+0.37(stat)-0.12+0.14(syst) ± 0.06(lumi) pb. This result is combined with a previous result from the ZZ → ℓ+ℓ-vv̄ channel resulting in a combined pp̄ → ZZ cross section measurement of 1.32-0.25+0.29(stat) ± 0.12(syst) ± 0.04(lumi) pb. These measurements are consistent with the standard model expectation of 1.43 ± 0.10 pb. We extend this analysis to search for the standard model (SM) Higgs boson between 115 and 200 GeV. At a Higgs boson mass of 125 GeV, we expect to set a limit of 43 times the SM expectation at 95% C.L., and set a limit of 42 times the SM expectation at 95% C.L. [ABSTRACT FROM AUTHOR]

We perform a search for the rare decay Bs0 → μ+μ- using data collected by the D0 experiment at the Fermilab Tevatron Collider. This result is based on the full D0 Run II data set corresponding to 10.4 fb-1 of pp̅ collisions at √s=1.96 TeV. We use a multivariate analysis to increase the sensitivity of the search. In the absence of an observed number of events above the expected background, we set an upper limit on the decay branching fraction of B(Bs0 → μ+μ-)<15×10-9 (12×10-9) at the 95% (90%) C.L. [ABSTRACT FROM AUTHOR]

We present an improved determination of the total width of the top quark, Гt, using 5.4 fb-1 of integrated luminosity collected by the D0 Collaboration at the Tevatron pp̅ Collider. The total width Гt is extracted from the partial decay width Г(t → Wb) and the branching fraction B(t → Wb). Г(i → Wb) is obtained from the t-channel single top-quark production cross section and B(t → Wb) is measured in tt̅ events. For a top mass of 172.5 GeV, the resulting width is Гt = 2.00+0.47-0.43 GeV. This translates to a top-quark lifetime of τt = (3.29+0.090-063) × 10-25 s. We also extract an improved direct limit on the Cabibbo-Kobayashi-Maskawa quark-mixing matrix element 0.81 < |Vtb| ≤ 1 at 95% C.L. and a limit of |Vtb′| < 0.59 for a high-mass fourth-generation bottom quark assuming unitarity of the fourth-generation quark-mixing matrix. [ABSTRACT FROM AUTHOR]