Search Results (519 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.

We report on transient reflectivity and magneto-optical Kerr measurements from the colossal magnetoresistance compounds La_0.67Ca_0.33MnO₃ (LCMO), La_0.67Sr_0.33MnO₃ (LSMO), and Sr₂FeMoO₆ (SFMO) as a function of temperature and magnetic field. In LCMO and LSMO, an unusually slow (∼1 μs) carrier relaxation component is revealed in the transient reflectivity traces. The component disappears as the transition temperature is approached from below. This slow decay process is attributed to spin-lattice relaxation of carriers in localized states and shows a close relationship with the spectral weight near the Fermi surface. The attribution is further supported by our pump-probe magneto-optical Kerr measurements. In addition to the clear observation of magnetic precessions, a long-lived exponentially decaying background reflects the spin-related relaxation of photoexcited electrons. In contrast to manganites, the temperature dependence of transient reflectivity is negligible, although there is a significant change in transient Kerr rotation in SFMO. Our results show that the dynamics of charge, spin, and lattice are strongly correlated with each other in the manganites, but the spin degree of freedom is thermally insulated from the electron and lattice systems.

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 report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

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.

The energetics of adsorbed hydrogen and surface germanium on vicinal silicon-germanium surfaces is important in understanding the growth of a silicon-germanium film at the atomic scale. By using plane wave pseudopotential density functional theory methods, we have calculated the molecular hydrogen adsorption energy at silicon-germanium sites at and near four different types of steps that are important. We have also calculated the relative energies for a germanium atom that is found at various sites in the vicinity of these steps. It is known that adsorbed hydrogen is more likely to be found at step sites than on terraces for pure silicon surfaces. However, our calculations show that the distribution of adsorbed hydrogen is considerably more uniform on silicon-germanium surfaces. Our calculations also show that for clean surfaces, germanium substitution is energetically favored at step sites, while for hydrogenated surfaces, the distribution of surface germanium atoms is likely to be uniform. These results show that the equilibrium distributions of adsorbed hydrogen and surface germanium atoms are correlated to each of their concentrations.

In this paper, the average density of states (ADOS) in graphene with binary alloy disorders is calculated by the recursion method. We observed an obvious resonant peak and a dip in ADOS curves near the Dirac point, which result from interactions with surrounding impurities. We also found that the resonance energy (E_r) and the dip position (ε_dip) are strongly dependent on the concentration of disorders (x) and their on-site potentials (v). A linear relation, ε_dip=xv, holds when the impurity concentration is low. This relation can also be extended when the impurity concentration is high, but with certain constraints. We also compute ADOS with a finite density of vacancies.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

It is shown that the data on the pion charge form factor admit the possibility for a substantial sea-quark component in the pion wave function. If the charge form factor is calculated with instant form kinematics in a constituent quark model that is extended to include explicit (qq̅ )² components in the pion wave function, that component will give the dominant contribution to the calculated π⁺ charge form factor at large values of momentum transfer. The present experimental values Q² can be described fairly well with (qq̅ )² component admixtures of up to 50%. The sensitivity of the calculated π⁺ charge form factor to whether one of the quarks or one of the antiquarks is taken to be in the P state is small.

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.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

The effects of spatiotemporally correlated noise on the regularity of spiking oscillations are studied in a network composed of Fitz-Hugh-Nagumo neurons. The spiking regularity of the neural network becomes the best at a moderate noise intensity, indicating the occurrence of coherence resonance (CR). The CR in a Watts-Strogatz small-world network is further improved by adding a small fraction of long-range connections. Given a set of temporal correlation constant τ and spatial correlation length λ of the noise, there exists an optimal network topology randomness, at which the spiking oscillations show the best regularity. The optimal randomness of the network topology at different τ and λ varies in a narrow range. Changing λ does not affect the optimal τ for achieving the most regular spike train, whereas varying τ, the best spiking regularity emerges at different optimal λ.

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.