Search Results (215 total)

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.

This Letter describes the search for a new heavy charged gauge boson W^′ decaying into an electron and a neutrino. The data were collected with the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV, and correspond to an integrated luminosity of about 1  fb^-1. Lacking any significant excess in the data in comparison with known processes, an upper limit is set on σ_W^′×B(W^′→eν), and a W^′ boson with mass below 1.00 TeV can be excluded at the 95% C.L., assuming standard-model-like couplings to fermions. This result significantly improves upon previous limits and is the most stringent to date.

We present a measurement of the muon charge asymmetry from W boson decays using 0.3  fb^-1 of data collected at sqrt[s]=1.96  GeV between 2002 and 2004 with the D0 detector at the Fermilab Tevatron pp̅ Collider. We compare our findings with expectations from next-to-leading-order calculations performed using the CTEQ6.1M and MRST04 NLO parton distribution functions. Our findings can be used to constrain future parton distribution function fits.

We present measurements of the process pp̅ →WZ+X→ℓ^′ν_ℓ^′ℓℓ̅ at sqrt[s]=1.96  TeV, where ℓ and ℓ^′ are electrons or muons. Using 1  fb^-1 of data from the D0 experiment, we observe 13 candidates with an expected background of 4.5±0.6 events and measure a cross section σ(WZ)=2.7_-1.3^+1.7  pb. From the number of observed events and the Z boson transverse momentum distribution, we limit the trilinear WWZ gauge couplings to -0.17≤λ_Z≤0.21(Δκ_Z=0) at the 95% C.L. for a form factor scale Λ=2  TeV. Further, assuming that Δg₁^Z=Δκ_Z, we find -0.12≤Δκ_Z≤0.29(λ_Z=0) at the 95% C.L. These are the most restrictive limits on the WWZ couplings available to date.

We report a study of the decay B_s⁰→D_s^(*)D_s^(*) using a data sample corresponding to 1.3  fb^-1 of integrated luminosity collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. One D_s^(*) meson was partially reconstructed in the decay D_s→ϕμν, and the other D_s^(*) meson was identified using the decay D_s→ϕπ where no attempt was made to distinguish D_s and D_s^* states. For the branching fraction Br(B_s⁰→D_s^(*)D_s^(*)) we obtain a 90% C.L. range [0.002,0.080] and central value 0.039_-0.017^+0.019(stat)_-0.015^+0.016(syst). This was subsequently used to make the most precise estimate of the width difference ΔΓ_s^CP in the B_s⁰-B̅ _s⁰ system: ΔΓ_s^CP/Γ_s=0.079_-0.035^+0.038(stat)_-0.030^+0.031(syst).

We present a measurement of the top quark pair production cross section in pp̅ collisions at sqrt[s]=1.96  TeV utilizing 425  pb^-1 of data collected with the D0 detector at the Fermilab Tevatron Collider. We consider the final state of the top quark pair containing one high-p_T electron or muon and at least four jets. We exploit specific kinematic features of tt̅ events to extract the cross section. For a top quark mass of 175 GeV, we measure σ_tt̅ =6.4_-1.2^+1.3(stat)±0.7(syst)±0.4(lum)  pb, in good agreement with the standard model prediction.

We search for the production of single top quarks via flavor-changing-neutral-current couplings of a gluon to the top quark and a charm (c) or up (u) quark. We analyze 230  pb^-1 of lepton+jets data from pp̅ collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters κ_g^c/Λ and κ_g^u/Λ, where κ_g define the strength of tcg and tug couplings, and Λ defines the scale of new physics. The limits at 95% C.L. are κ_g^c/Λ<0.15  TeV^-1 and κ_g^u/Λ<0.037  TeV^-1.

We report results from a search for the decay B_s⁰→μ⁺μ^- using 1.3  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the D0 experiment at the Fermilab Tevatron Collider. We find two candidate events, consistent with the expected background of 1.24±0.99, and set an upper limit on the branching fraction of B(B_s⁰→μ⁺μ^-)<1.2×10^-7 at the 95% C.L.

We report a measurement of the Λ_b⁰ lifetime using a sample corresponding to 1.3  fb^-1 of data collected by the D0 experiment in 2002–2006 during run II of the Fermilab Tevatron collider. The Λ_b⁰ baryon is reconstructed via the decay Λ_b⁰→μν̅ Λ_c⁺X. Using 4437±329 signal candidates, we measure the Λ_b⁰ lifetime to be τ(Λ_b⁰)=1.290_-0.110^+0.119(stat)_-0.091^+0.087(syst)  ps, which is among the most precise measurements in semileptonic Λ_b⁰ decays. This result is in good agreement with the world average value.

This Letter presents the first strong evidence for the resolution of the excited B mesons B₁ and B₂^* as two separate states in fully reconstructed decays to B^+(*)π^-. The mass of B₁ is measured to be 5720.6±2.4±1.4  MeV/c² and the mass difference ΔM between B₂^* and B₁ is 26.2±3.1±0.9  MeV/c², giving the mass of the B₂^* as 5746.8±2.4±1.7  MeV/c². The production rate for B₁ and B₂^* mesons is determined to be a fraction (13.9±1.9±3.2)% of the production rate of the B⁺ meson.

A measurement of the top quark pair production cross section in proton antiproton collisions at an interaction energy of sqrt[s]=1.96  TeV is presented. This analysis uses 405±25  pb^-1 of data collected with the D0 detector at the Fermilab Tevatron Collider. Fully hadronic tt̅ decays with final states of six or more jets are separated from the multijet background using secondary vertex tagging and a neural network. The tt̅ cross section is measured as σ_tt̅ =4.5_-1.9^+2.0(stat)_-1.1^+1.4(syst)±0.3(lumi)  pb for a top quark mass of m_t=175  GeV/c².

We have measured the Λ_b lifetime using the exclusive decay Λ_b→J/ψΛ, based on 1.2  fb^-1 of data collected with the D0 detector during 2002–2006. From 171 reconstructed Λ_b decays, where the J/ψ and Λ are identified via the decays J/ψ→μ⁺μ^- and Λ→pπ, we measured the Λ_b lifetime to be τ(Λ_b)=1.218_-0.115^+0.130(stat)±0.042(syst)  ps. We also measured the B⁰ lifetime in the decay B⁰→J/ψ(μ⁺μ^-)K_S⁰(π⁺π^-) to be τ(B⁰)=1.501_-0.074^+0.078(stat)±0.050(syst)  ps, yielding a lifetime ratio of τ(Λ_b)/τ(B⁰)=0.811_-0.087^+0.096(stat)±0.034(syst).

We present a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.96  TeV utilizing approximately 425  pb^-1 of data collected with the D0 detector. We consider decay channels containing two high p_T charged leptons (either e or μ) from leptonic decays of both top-daughter W bosons. These were gathered using four sets of selection criteria, three of which required that a pair of fully identified leptons (i.e., eμ, ee, or μμ) be found. The fourth approach imposed less restrictive criteria on one of the lepton candidates and required that at least one hadronic jet in each event be tagged as containing a b quark. For a top quark mass of 175 GeV, the measured cross section is 7.4±1.4(stat)±1.0(syst)  pb and for the current Tevatron average top quark mass of 170.9 GeV, the resulting value of the cross section is 7.8±1.8(stat+syst)  pb.

Long-lived, heavy particles are predicted in a number of models beyond the standard model of particle physics. We present the first direct search for such particles’ decays, occurring up to 100 h after their production and not synchronized with an accelerator bunch crossing. We apply the analysis to the gluino (g˜), predicted in split supersymmetry, which after hadronization can become charged and lose enough momentum through ionization to come to rest in dense particle detectors. Approximately 410  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected with the D0 detector during Run II of the Fermilab Tevatron collider are analyzed in search of such “stopped gluinos” decaying into a gluon and a neutralino (χ˜₁⁰). Limits are placed on the (gluino cross section)×(probability to stop)×[BR(g˜→gχ˜₁⁰)] as a function of the gluino and χ˜₁⁰ masses, for gluino lifetimes from 30  μs–100  h.