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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 D0 experiment during Run II of the Fermilab Tevatron Collider, corresponding to 9.7 inverse fb of integrated luminosity, is used. The asymmetry measured for reconstructed leptons is $A_{FB}^l = \big(2.9 \pm 2.1(stat.) ^{+1.5}_{-1.7}(syst.) \big)$%. When corrected for efficiency and resolution effects within the lepton rapidity coverage of $|y_l|<1.5$, the asymmetry is found to be $A_{FB}^l = \big(4.2 \pm 2.3(stat.) ^{+1.7}_{-2.0}(syst.) \big)$%. Combination with the asymmetry measured in the dilepton final state yields $A_{FB}^l = \big(4.2 \pm 2...
Single top quark production via four-fermion contact interactions associated to flavour-changing neutral currents was searched for in data taken by the DELPHI detector at LEP2. The data were accumulated at centre-of-mass energies ranging from 189 to 209 GeV, with an integrated luminosity of 598.1 pb^-1. No evidence for a signal was found. Limits on the energy scale Lambda, were set for scalar-, vector- and tensor-like coupling scenarios.
We perform a search for the rare decay Bs->mumu using data collected by the D0 experiment at the Fermilab Tevatron Collider. This result is based on the full D0 Run II dataset corresponding to 10.4 fb^(-1) of ppbar collisions at sqrt(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 BR(Bs->mumu) < 15x10^(-9) at the 95% C.L.
We present the first combined measurement of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations, based on the recently proposed quantity $R_{\Delta \phi}$. The variable $R_{\Delta \phi}$ measures the fraction of the inclusive dijet events in which the azimuthal separation of the two jets with the highest transverse momenta is less than a specified value for the parameter $\Delta \phi_{\rm max}$. The quantity $R_{\Delta \phi}$ is measured in $p\bar{p}$ collisions at $\sqrt{s}=1.96\,$TeV, as a function of the dijet rapidity interval, the total scalar transverse momentum, and $\Delta \phi_{\rm max}$. The measurement uses an event sample corresponding to an integrated luminosity of $0.7\,$fb$^{-1}$ collected with the D0 detector at the Fermilab Tevatron Collider. The results are compared to predictions of a p...
We present measurements of the differential cross section $d\sigma/dp_{T}^{\gamma}$ for the associated production of a $c$-quark jet and an isolated photon with rapidity $|y^{\gamma}|< 1.0$ and transverse momentum $30 < p_{T}^{\gamma} < 300$ GeV. The $c$-quark jets are required to have $|y^{jet}| < 1.5$ and $p_{T}^{jet} >15$ GeV. The cross section ratios $\sigma(\gamma+c)/\sigma(\gamma+b)$ as a function of $p_{T}^{\gamma}$ are also presented. The results are based on data corresponding to an integrated luminosity of 8.7 fb$^{-1}$ recorded with the D0 detector at the Fermilab Tevatron $p\bar{p}$ Collider at $\sqrt{s}=$1.96 TeV. The obtained results are compared to next-to-leading order perturbative QCD calculations using various parton distribution functions, to predictions based on the $k_{T}$-factorization approach, and to predictions...
We present measurements of direct photon pair production cross sections using 8.5 fb$^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron $p \bar p$ collider. The results are presented as differential distributions of the photon pair invariant mass $d\sigma/dM_{\gamma \gamma}$, pair transverse momentum $d \sigma /dp^{\gamma \gamma}_T$, azimuthal angle between the photons $d\sigma/d\Delta \phi_{\gamma \gamma}$, and polar scattering angle in the Collins-Soper frame $d\sigma /d|\cos \theta^*|$. Measurements are performed for isolated photons with transverse momenta $p^{\gamma}_T>18 ~(17)$ GeV for the leading (next-to-leading) photon in $p_T$, pseudorapidities $|\eta^{\gamma}|<0.9$, and a separation in $\eta-\phi$ space $\Delta\mathcal R_{\gamma\gamma} > 0.4$. We present comparisons with the predictions from Monte Carlo e...
We present a search for the standard model (SM) Higgs boson produced in association with a Z boson in 9.7 fb-1 of ppbar collisions collected with the D0 detector at the Fermilab Tevatron Collider at sqrt{s} = 1.96 TeV. Selected events contain one reconstructed Z -> e+e- or Z -> mu+mu- candidate and at least two jets, including at least one jet likely to contain a b quark. To validate the search procedure, we also measure the cross section for ZZ production, and find that it is consistent with the SM expectation. We set upper limits at the 95% C.L. on the product of the ZH production cross section and branching ratio B(H -> bbbar) for Higgs boson masses 90 <= M_H <= 150 GeV. The observed (expected) limit for M_H = 125 GeV is a factor of 7.1 (5.1) larger than the SM prediction.
We perform a combination of searches for standard model Higgs boson production in $p\bar{p}$ collisions recorded by the D0 detector at the Fermilab Tevatron Collider at a center of mass energy of $\sqrt{s}=1.96$ TeV. The different production and decay channels have been analyzed separately, with integrated luminosities of up to 9.7 fb$^{-1}$ and for Higgs boson masses $90\leq M_H \leq 200$ GeV. We combine these final states to achieve optimal sensitivity to the production of the Higgs boson. We also interpret the combination in terms of models with a fourth generation of fermions, and models with suppressed Higgs boson couplings to fermions. The result excludes a standard model Higgs boson at 95% C.L. in the ranges $90 < M_H < 101$ GeV and $157
We present a search for Higgs bosons in multilepton final states in pp-bar collisions at sqrt(s)=1.96 TeV recorded with the D0 detector at the Fermilab Tevatron Collider, using the full Run II data set with integrated luminosities of up to 9.7 fb-1. The multilepton states considered are two electron plus muon, electron with two muons, muon with two hadronic tau leptons, and like-charge electron-muon pairs. These channels directly probe the HVV (V=W,Z) coupling of the Higgs boson in production and decay. The muon with two hadronic tau lepton channel is also sensitive to H to tau lepton pair decays. Upper limits at the 95% C.L on the rate of standard model Higgs boson production are derived in the mass range 100 <= M_H <= 200 GeV. The expected and observed limits are a factor of 6.3 and 8.4 above the predicted standard model cross sectio...
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