Search Results (117 total)

We present a search for the production of neutral Higgs bosons ϕ decaying into τ⁺τ^- final states in pp̅ collisions at a center-of-mass energy of 1.96 TeV. The data, corresponding to an integrated luminosity of approximately 1  fb^-1, were collected by the D0 experiment at the Fermilab Tevatron Collider. Limits on the production cross section times branching ratio are set. The results are interpreted in the minimal supersymmetric standard model yielding limits that are the most stringent to date at hadron colliders.

We report the results of a search for pair production of doubly charged Higgs bosons via pp̅ →H⁺⁺H^--X→μ⁺μ⁺μ^-μ^-X at sqrt[s]=1.96  TeV. We use a data set corresponding to an integrated luminosity of 1.1  fb^-1 collected from 2002 to 2006 by the D0 detector at the Fermilab Tevatron Collider. In the absence of an excess above the standard model background, lower mass limits of M(H_L^±±)>150  GeV/c² and M(H_R^±±)>127  GeV/c² at 95% C.L. are set, respectively, for left-handed and right-handed doubly charged Higgs bosons assuming a 100% branching ratio into muons.

We report on a measurement of the inclusive jet cross section in pp̅ collisions at a center-of-mass energy sqrt[s]=1.96  TeV using data collected by the D0 experiment at the Fermilab Tevatron Collider corresponding to an integrated luminosity of 0.70  fb^-1. The data cover jet transverse momenta from 50 to 600 GeV and jet rapidities in the range -2.4 to 2.4. Detailed studies of correlations between systematic uncertainties in transverse momentum and rapidity are presented, and the cross section measurements are found to be in good agreement with next-to-leading order QCD calculations.

The energy spectrum of cosmic rays above 2.5×10¹⁸ eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index γ of the particle flux, J∝E^-γ, at energies between 4×10¹⁸ eV and 4×10¹⁹ eV is 2.69±0.02(stat)±0.06(syst), steepening to 4.2±0.4(stat)±0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz’min.

We report the results of a search for a narrow resonance decaying into two photons in 1.1  fb^-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider during the period 2002–2006. We find no evidence for such a resonance and set a lower limit on the mass of a fermiophobic Higgs boson of m_hf>100  GeV at the 95% C.L. This exclusion limit exceeds those obtained in previous searches at the Fermilab Tevatron and covers a significant region of the parameter space B(h_f→γγ) vs m_hf which was not accessible at the CERN Large Electron-Positron Collider.

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron pp̅ collider. The standard model predicts that the electroweak interaction can produce a top quark together with an antibottom 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 tt̅ 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(pp̅ →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 |V_tbf₁^L|=1.31_-0.21^+0.25, where f₁^L is a generic vector coupling. This model-independent measurement translates into 0.68<|V_tb|≤1 at the 95% C.L. in the standard model.

A fully reconstructed B_c→J/ψπ signal is observed with the D0 detector at the Fermilab Tevatron pp̅ collider using 1.3  fb^-1 of integrated luminosity. The signal consists of 54±12 candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the B_c meson is 6300±14(stat)±5(syst)  MeV/c².

We report on a search for large extra dimensions in a data sample of approximately 1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV. We investigate Kaluza-Klein graviton production with a photon and missing transverse energy in the final state. At the 95% C.L. we set limits on the fundamental mass scale M_D from 884 to 778 GeV for two to eight extra dimensions.

We present results from a study of pp̅ →Wγ+X events utilizing data corresponding to 0.7  fb^-1 of integrated luminosity at sqrt[s]=1.96  TeV collected by the D0 detector at the Fermilab Tevatron Collider. We set limits on anomalous WWγ couplings at the 95% C.L. The one-dimensional 95% C.L. limits are 0.49<κ_γ<1.51 and -0.12<λ_γ<0.13. We make the first study of the charge-signed rapidity difference between the lepton and the photon and find it to be indicative of the standard model radiation-amplitude zero in the Wγ system.

We report a search for R-parity-violating production and decay of sneutrino particles in the eμ final state with 1.04±0.06  fb^-1 of data collected with the D₀ detector at the Fermilab Tevatron Collider in 2002–2006. Good agreement between the data and the standard model prediction is observed. With no evidence for new physics, we set limits on the R-parity-violating couplings λ₃₁₁^′ and λ₃₁₂ as a function of the sneutrino mass.

We search for the production of a heavy W^′ gauge boson that decays to third generation quarks in 0.9  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV, collected with the D0 detector at the Fermilab Tevatron collider. We find no significant excess in the final-state invariant mass distribution and set upper limits on the production cross section times branching fraction. For a left-handed W^′ boson with SM couplings, we set a lower mass limit of 731 GeV. For right-handed W^′ bosons, we set lower mass limits of 739 GeV if the W^′ boson decays to both leptons and quarks and 768 GeV if the W^′ boson decays only to quarks. We also set limits on the coupling of the W^′ boson to fermions as a function of its mass.

We present a search for direct CP violation in B^±→J/ψK^±(π^±) decays. The event sample is selected from 2.8  fb^-1 of pp̅ collisions recorded by D0 experiment in run II of the Fermilab Tevatron Collider. The charge asymmetry A_CP(B⁺→J/ψK⁺)=+0.0075±0.0061(stat)±0.0030(syst) is obtained using a sample of approximately 40 000 B^±→J/ψK^± decays. The achieved precision is of the same level as the expected deviation predicted by some extensions of the standard model. We also measured the charge asymmetry A_CP(B⁺→J/ψπ⁺)=-0.09±0.08(stat)±0.03(syst).

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth’s crust. Tau leptons from ν_τ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ν_τ at EeV energies. Assuming an E_ν^-2 differential energy spectrum the limit set at 90% C.L. is E_ν²dN_ντ/dE_ν<1.3×10^-7  GeV cm^-2 s^-1 sr^-1 in the energy range 2×10¹⁷  eV<E_ν<2×10¹⁹  eV.

We measure the tt̅ production cross section in pp̅ collisions at sqrt[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 σ_tt̅ =7.62±0.85  pb, assuming the current world average m_t=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.

We present the first simultaneous measurement of the ratio of branching fractions, R=B(t→Wb)/B(t→Wq), with q being a d, s, or b quark, and the top-quark pair production cross section σ_tt̅ in the lepton plus jets channel using 0.9  fb^-1 of pp̅ collision data at sqrt[s]=1.96  TeV collected with the D0 detector. We extract R and σ_tt̅ by analyzing samples of events with 0, 1, and ≥2 identified b jets. We measure R=0.97_-0.08^+0.09(stat+syst) and σ_tt̅ =8.18_-0.84^+0.90(stat+syst)±0.50(lumi)  pb, in agreement with the standard model prediction.

We present the results of a search for the production of an excited state of the electron, e^*, in proton-antiproton collisions at sqrt[s]=1.96  TeV. The data were collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 1  fb^-1. We search for e^* in the process pp̅ →e^*e, with the e^* subsequently decaying to an electron plus photon. No excess above the standard model background is observed. Interpreting our data in the context of a model that describes e^* production by four-fermion contact interactions and e^* decay via electroweak processes, we set 95% C.L. upper limits on the production cross section ranging from 8.9 to 27 fb, depending on the mass of the excited electron. Choosing the scale for contact interactions to be Λ=1  TeV, excited electron masses below 756 GeV are excluded at the 95% C.L.

We present the first measurement of the integrated forward-backward charge asymmetry in top-quark–top-antiquark pair (tt̅ ) production in proton-antiproton (pp̅ ) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming tt̅ +X production and decay, a sample of 0.9  fb^-1 of data, collected by the D0 experiment at the Fermilab Tevatron Collider, is used to measure the asymmetry for different jet multiplicities. The result is also used to set upper limits on tt̅ +X production via a Z^′ resonance.

We present a study of μμμμ, eeee, and μμee events using 1  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV. Requiring the lepton pair masses to be greater than 30 GeV, we observe one event, consistent with the expected background of 0.13±0.03 events and with the predicted standard model ZZ and Zγ^* production of 1.71±0.15 events. We set an upper limit on the ZZ and Zγ^* cross section of 4.4 pb at the 95% C.L. We also derive limits on anomalous neutral trilinear ZZZ and ZZγ^* gauge couplings. The one-parameter 95% C.L. coupling limits with a form-factor scale Λ=1.2  TeV are -0.28<f₄₀^Z<0.28, -0.31<f₅₀^Z<0.29, -0.26<f₄₀^γ<0.26, and -0.30<f₅₀^γ<0.28.

We study the flavor-changing-neutral-current process c→uμ⁺μ^- using 1.3  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV recorded by the D0 detector operating at the Fermilab Tevatron Collider. We see clear indications of the charged-current mediated D_s⁺ and D⁺→ϕπ⁺→μ⁺μ^-π⁺ final states with significance greater than 4 standard deviations above background for the D⁺ state. We search for the continuum neutral-current decay of D⁺→π⁺μ⁺μ^- in the dimuon invariant mass spectrum away from the ϕ resonance. We see no evidence of signal above background and set a limit of B(D⁺→π⁺μ⁺μ^-)<3.9×10^-6 at the 90% C.L. This limit places the most stringent constraint on new phenomena in the c→uμ⁺μ^- transition.

We present a measurement of the shape of the Z/γ^* boson transverse momentum (q_T) distribution in pp̅ →Z/γ^*→e⁺e^-+X events at a center-of-mass energy of 1.96 TeV using 0.98  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron collider. The data are found to be consistent with the resummation prediction at low q_T, but above the perturbative QCD calculation in the region of q_T>30  GeV/c. Using events with q_T<30  GeV/c, we extract the value of g₂, one of the nonperturbative parameters for the resummation calculation. Data at large boson rapidity y are compared with the prediction of resummation and with alternative models that employ a resummed form factor with modifications in the small Bjorken x region of the proton wave function.

We search for decays of Kaluza-Klein excitations of the graviton in the Randall-Sundrum model of extra dimensions to e⁺e^- and γγ in 1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the D0 detector at the Fermilab Tevatron. We set 95% confidence level upper limits on the production cross section times branching fraction, which translate into lower limits on the mass of the lightest excitation between 300 and 900 GeV for values of the coupling k/M̅ _Pl between 0.01 and 0.1.

We report the direct observation of the excited L=1 state B_s2^* in fully reconstructed decays to B⁺K^-. The mass of the B_s2^* meson is measured to be 5839.6±1.1(stat)±0.7(syst)  MeV/c², and its production rate relative to the B⁺ meson is measured to be [1.15±0.23(stat)±0.13(syst)]%.

We present the first model-independent measurement of the helicity of W bosons produced in top quark decays, based on a 1  fb^-1 sample of candidate tt̅ events in the dilepton and lepton plus jets channels collected by the D0 detector at the Fermilab Tevatron pp̅ Collider. We reconstruct the angle θ^* between the momenta of the down-type fermion and the top quark in the W boson rest frame for each top quark decay. A fit of the resulting cos⁡θ^* distribution finds that the fraction of longitudinal W bosons f₀=0.425±0.166(stat)±0.102(syst) and the fraction of right-handed W bosons f₊=0.119±0.090(stat)±0.053(syst), which is consistent at the 30% C.L. with the standard model.