Search Results (240 total)

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).

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 present the results of a search for short-duration gravitational-wave bursts associated with 39 gamma-ray bursts (GRBs) detected by gamma-ray satellite experiments during LIGO’s S2, S3, and S4 science runs. The search involves calculating the crosscorrelation between two interferometer data streams surrounding the GRB trigger time. We search for associated gravitational radiation from single GRBs, and also apply statistical tests to search for a gravitational-wave signature associated with the whole sample. For the sample examined, we find no evidence for the association of gravitational radiation with GRBs, either on a single-GRB basis or on a statistical basis. Simulating gravitational-wave bursts with sine-Gaussian waveforms, we set upper limits on the root-sum-square of the gravitational-wave strain amplitude of such waveforms at the times of the GRB triggers. We also demonstrate how a sample of several GRBs can be used collectively to set constraints on population models. The small number of GRBs and the significant change in sensitivity of the detectors over the three runs, however, limits the usefulness of a population study for the S2, S3, and S4 runs. Finally, we discuss prospects for the search sensitivity for the ongoing S5 run, and beyond for the next generation of detectors.

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 on a search for gravitational waves from the coalescence of compact binaries during the third and fourth LIGO science runs. The search focused on gravitational waves generated during the inspiral phase of the binary evolution. In our analysis, we considered three categories of compact binary systems, ordered by mass: (i) primordial black hole binaries with masses in the range 0.35M_⊙<m₁, m₂<1.0M_⊙, (ii) binary neutron stars with masses in the range 1.0M_⊙<m₁, m₂<3.0M_⊙, and (iii) binary black holes with masses in the range 3.0M_⊙<m₁, m₂<m_max⁡ with the additional constraint m₁+m₂<m_max⁡, where m_max⁡ was set to 40.0M_⊙ and 80.0M_⊙ in the third and fourth science runs, respectively. Although the detectors could probe to distances as far as tens of Mpc, no gravitational-wave signals were identified in the 1364 hours of data we analyzed. Assuming a binary population with a Gaussian distribution around 0.75-0.75M_⊙, 1.4-1.4M_⊙, and 5.0-5.0M_⊙, we derived 90%-confidence upper limit rates of 4.9  yr^-1L₁₀^-1 for primordial black hole binaries, 1.2  yr^-1L₁₀^-1 for binary neutron stars, and 0.5  yr^-1L₁₀^-1 for stellar mass binary black holes, where L₁₀ is 10¹⁰ times the blue-light luminosity of the Sun.

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.