Search Results (1,570 total)

We report bulk magnetization, and elastic and inelastic neutron scattering measurements under an external magnetic field H on the weakly coupled distorted kagome system, Cu₂(OD)₃Cl. Our results show that the ordered state below 6.7 K is a canted antiferromagnet and consists of large antiferromagnetic ac components and smaller ferromagnetic b components. By first-principles calculations and linear spin wave analysis, we present a simple spin Hamiltonian with nonuniform nearest neighbor exchange interactions resulting in a system of coupled spin trimers with a single-ion anisotropy that can qualitatively reproduce the spin dynamics of Cu₂(OD)₃Cl.

Using the previous Belle measurement of the inclusive photon energy in B→X_sγ decays, we determine the first and second moments of this spectrum for minimum photon energies in the B meson rest frame ranging from 1.8 to 2.3 GeV. Combining these measurements with recent Belle data on the lepton energy and hadronic mass moments in B→X_cℓν decays, we perform fits to theoretical expressions derived in the 1S and kinetic mass schemes and extract the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V_cb, the b-quark mass, and other nonperturbative parameters. In the 1S scheme analysis we find |V_cb|=(41.56±0.68(fit)±0.08(τ_B))×10^-3 and m_b^1S=(4.723±0.055)  GeV. In the kinetic scheme, we obtain |V_cb|=(41.58±0.69(fit)±0.08(τ_B)±0.58(th))×10^-3 and m_b^kin=(4.543±0.075)  GeV.

The recently discovered superconductor LaO_0.87F_0.13FeAs (T_C≈26 K) was investigated using the neutron-scattering technique. No spin-density-wave (SDW) order was observed in the normal state or in the superconducting state, both with and without an applied magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraint on the spin dynamic response from a d-wave pairing, an upper limit for the magnetic-resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measurements also support the energy scale of the calculated phonon spectrum, which is used in the electron-phonon coupling theory and fails to produce the high observed T_C.

We present the results of the first hadron collider search for heavy, long-lived neutralinos that decay via χ˜₁⁰→γG˜ in gauge-mediated supersymmetry breaking models. Using an integrated luminosity of 570±34  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV, we select γ+jet+missing transverse energy candidate events based on the arrival time of a high-energy photon at the electromagnetic calorimeter as measured with a timing system that was recently installed on the CDF II detector. We find 2 events, consistent with the background estimate of 1.3±0.7 events. While our search strategy does not rely on model-specific dynamics, we set cross section limits and place the world-best 95% C.L. lower limit on the χ˜₁⁰ mass of 101  GeV/c² at τ_χ˜1⁰=5  ns.

We present a search for standard model Higgs boson production in association with a W boson in proton-antiproton collisions (pp̅ →W^±H→ℓνbb̅ ) at a center of mass energy of 1.96 TeV. The search employs data collected with the CDF II detector which correspond to an integrated luminosity of approximately 1  fb^-1. We select events consistent with a signature of a single lepton (e^±/μ^±), missing transverse energy, and two jets. Jets corresponding to bottom quarks are identified with a secondary vertex tagging method and a neural network filter technique. The observed number of events and the dijet mass distributions are consistent with the standard model background expectations, and we set 95% confidence level upper limits on the production cross section times branching ratio ranging from 3.9 to 1.3 pb for Higgs boson masses from 110 to 150  GeV/c², respectively.

We search for pair production of supersymmetric top quarks (t˜₁), followed by R-parity violating decay t˜₁→τb with a branching ratio β, using 322  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the upgraded Collider Detector at Fermilab. Two candidate events pass our final selection criteria, consistent with the standard model expectation. We set upper limits on the cross section σ(t˜₁t˜̅ ₁)×β² as a function of the top-squark mass m(t˜₁). Assuming β=1, we set a 95% confidence level limit m(t˜₁)>153  GeV/c². The limits are also applicable to the case of a third-generation scalar leptoquark (LQ₃) decaying LQ₃→τb.

The distribution of magnetic domain pinning fields was determined in ferromagnetic GaMnAs films using the angular dependence of the planar Hall effect. A major difference is found between the pinning field distribution in as-grown and in annealed films: the former showing a strikingly narrower distribution than the latter. This effect, which we ascribe to differences in the degree of uniformity of magnetic anisotropy, provides a better understanding of magnetic domain landscape in GaMnAs, a subject of current intense interest.

The reaction π^-p→ηn is investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson baryon channels included are πN,ηN,πΔ,σN, and ρN. The nonresonant meson-baryon interactions of the model are derived from a set of Lagrangians by using a unitary transformation method. One or two excited nucleon states in each of S,P,D, and F partial waves are included to generate the resonant amplitudes. Data of the π^-p→ηn reaction from threshold up to a total center-of-mass energy of about 2 GeV are satisfactorily reproduced and the roles played by the following nine nucleon resonances are investigated: S₁₁(1535),S₁₁(1650),P₁₁(1440),P₁₁(1710),P₁₃(1720),D₁₃(1520),D₁₃(1700),D₁₅(1675), and F₁₅(1680). The reaction mechanism and the predicted ηN scattering length are discussed.

At high area fractions, monolayers of colloidal dimer particles form a degenerate crystal (DC) structure in which the particle lobes occupy triangular lattice sites while the particles are oriented randomly along any of the three lattice directions. We report that dislocation glide in DCs is blocked by certain particle orientations. The mean number of lattice constants between such obstacles is Z̅ _exp⁡=4.6±0.2 in experimentally observed DC grains and Z̅ _sim=6.18±0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these obstacles is observed to proceed through dislocation reactions. We estimate that the energetic cost of dislocation pair separation via such reactions in an otherwise defect free DC grows linearly with final separation, hinting that the material properties of DCs may be dramatically different from those of 2-D crystals of spheres.

We investigated the magnetic structure and short-range spin correlations of multiferroic hexagonal manganite YMnO₃ by powder neutron diffraction at high pressures up to 6.7 GPa. High pressure induces a spin-liquid phase in multiferroic YMnO₃, coexisting with the suppressed long-range antiferromagnetic order. This spin-liquid phase exhibits a temperature dependence distinctively different from short-range spin correlations seen at ambient pressure. Its formation occurs through an in-plane Mn-O bond symmetrization and results in reduced magnetoelastic coupling at high pressures.

Equilibrium and nonequilibrium relaxation behaviors of two-dimensional superconducting arrays are investigated via numerical simulations at low temperatures in the presence of incommensurate transverse magnetic fields, with frustration parameter f=(3−sqrt[5])/2. We find that the nonequilibrium relaxation, beginning with random initial states quenched to low temperatures, exhibits a three-stage relaxation of chirality autocorrelations. At the early stage, the relaxation is found to be described by the von Schweidler form. Then it exhibits power-law behavior in the intermediate-time scale and faster decay in the long-time limit, which together can be fitted to the Ogielski form; for longer waiting times, this crosses over to a stretched exponential form. We argue that the power-law behavior in the intermediate-time scale may be understood as a consequence of the coarsening behavior, leading to the local vortex order corresponding to f=2/5 ground-state configurations. High mobility of the vortices in the domain boundaries, generating slow wandering motion of the domain walls, may provide mechanism of dynamic heterogeneity and account for the long-time stretched exponential relaxation behavior. It is expected that such meandering fluctuations of the low-temperature structure give rise to finite resistivity at those low temperatures; this appears consistent with the zero-temperature resistive transition in the limit of irrational frustration.

We report the results of a search for the anomalous production of a massive particle decaying to four electrons via two Z⁰ bosons in 1.1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range 500–1000  GeV/c² to be 0.028±0.009(stat)±0.011(syst) events. We observe zero events in this search region. Assuming a Randall-Sundrum graviton production model, we set 95% C.L. limits on σ×BF(G→Z⁰Z⁰)<4–6  pb, depending on the graviton mass.

In this paper, we investigate what kinds of quantum states are feasible to perform perfectly secure secret sharing, and present its necessary and sufficient conditions. We also show that the states are bipartite distillable for all bipartite splits, and hence the states could be distillable into the Greenberger-Horne-Zeilinger state. We finally exhibit a class of secret-sharing states, which have an arbitrarily small amount of bipartite distillable entanglement for a certain split.

We report on a search for the doubly Cabibbo suppressed decay B⁺→D^*+π⁰, based on a data sample of 657×10⁶ BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We find no significant signal and set an upper limit of B(B⁺→D^*+π⁰)<3.6×10^-6 at the 90% confidence level. This limit can be used to constrain the ratio between suppressed and favored B→D^*π decay amplitudes, r<0.051, at the 90% confidence level.

We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are used to rapidly and locally heat the substrate at the buried interface of an Al_0.3Ga_0.7As/GaAs heterostructure grown by molecular-beam epitaxy. High-resolution time-resolved x-ray diffraction is used to study the heating and cooling of the film and substrate independently. The data are compared with a simple model for the thermal transport incorporated into dynamical diffraction calculations allowing us to extract the room-temperature cross-plane film thermal conductivity. The value is 40% lower than that extrapolated from prior results on liquid-phase epitaxy grown samples of varying concentrations.

We investigate photo-double-ionization of H₂ by circular polarized photons at hν=160  eV. The previously observed two-center interference pattern in the angular distribution of the electron sum momentum is shown to depend strongly on the internuclear distance and the summed electron momenta.

We present a new method for studying high-p_T dilepton events (e^±e^∓, μ^±μ^∓, e^±μ^∓) and simultaneously extracting the production cross sections of pp̅ →tt̅ , pp̅ →W⁺W^-, and pp̅ →Z⁰→τ⁺τ^- at a center-of-mass energy of sqrt[s]=1.96  TeV. We perform a likelihood fit to the dilepton data in a parameter space defined by the missing transverse energy and the number of jets in the event. Our results, which use 360  pb^-1 of data recorded with the CDF II detector at the Fermilab Tevatron Collider, are σ(tt̅ )=8.5_-2.2^+2.7  pb, σ(W⁺W^-)=16.3_-4.4^+5.2  pb, and σ(Z⁰→τ⁺τ^-)=291_-46⁺⁵⁰  pb.

The growth behavior of small self-interstitial clusters in crystalline Si is presented based on extensive combined Metropolis Monte Carlo, tight-binding molecular dynamics, and density-functional theory calculations. New stable structures for small interstitial clusters (I_n,5≤n≤16) are determined, showing that the compact geometry appears favored when the cluster size is smaller than 10 atoms (n<10). The fourfold-coordinated dodecainterstitial (I₁₂) structure with C_2h symmetry is identified to serve as an effective nucleation center for larger extended defects. This work provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around n=10.

Data collected in run II of the Fermilab Tevatron are searched for indications of new electroweak scale physics. Rather than focusing on particular new physics scenarios, CDF data are analyzed for discrepancies with respect to the standard model prediction. A model-independent approach (Vista) considers the gross features of the data and is sensitive to new large cross section physics. A quasi-model-independent approach (Sleuth) searches for a significant excess of events with large summed transverse momentum and is particularly sensitive to new electroweak scale physics that appears predominantly in one final state. This global search for new physics in over 300 exclusive final states in 927  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV reveals no such significant indication of physics beyond the standard model.

The magneto-optical interpolarization conversions by a layer of quantum dots have been investigated. Various types of polarization response of the sample were observed as a function of external magnetic field and of the orientation of the sample. The full set of experimental dependences is analyzed in terms of a one-step and a two-step model of spin evolution. The angular distribution of the quantum dots over the directions of elongation in the plane of the sample is taken into account in terms of the two models, and the model predictions are compared with experimental observations.

Using a 492  fb^-1 data sample collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider, we observe the decay B⁰→pp̅ K^*0 with a branching fraction of (1.18_-0.25^+0.29(stat)±0.11(syst))×10^-6. We study the decay dynamics of B⁰→pp̅ K^*0 and compare with B⁺→pp̅ K^*+. The K^*0 meson is found to be almost 100% polarized (with a fraction of (101±13±3)% in the helicity zero state), while the K^*+ meson has a (32±17±9)% fraction in the helicity zero state. The direct CP asymmetries for B⁰→pp̅ K^*0 and B⁺→pp̅ K^*+ are measured to be -0.08±0.20±0.02 and -0.01±0.19±0.02, respectively. In addition, we report improved measurements of the branching fractions B(B⁺→pp̅ K^*+)=(3.38_-0.60^+0.73±0.39)×10^-6 and B(B⁰→pp̅ K⁰)=(2.51_-0.29^+0.35±0.21)×10^-6, which supersede our previous measurements.

We present a measurement of the branching fraction B(D_s⁺→μ⁺ν_μ) using a 548  fb^-1 data sample collected by the Belle experiment at the KEKB e⁺e^- collider. The D_s momentum is determined by reconstruction of the system recoiling against DKγX in events of the type e⁺e^-→D_s^*DKX, D_s^*→D_sγ, where X represents additional pions or photons from fragmentation. This full-reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that achieved by experiments at the tau-charm factories. We obtain the branching fraction B(D_s⁺→μ⁺ν_μ)=[6.44±0.76(stat)±0.57(syst)]×10^-3, implying a D_s decay constant of f_Ds=[275±16(stat)±12(syst)]  MeV.

A search for mixing in the neutral D meson system has been performed using semileptonic D⁰→K^(*)-e⁺ν and D⁰→K^(*)-μ⁺ν decays. Neutral D mesons from D^*+→D⁰π_s⁺ decays are used and the flavor at production is tagged by the charge of the slow pion. The measurement is performed using 492  fb^-1 of data recorded by the Belle detector. From the yield of right-sign and wrong-sign decays arising from nonmixed and mixed events, respectively, we measure the ratio of the time-integrated mixing rate to the unmixed rate to be R_M=(1.3±2.2±2.0)×10^-4. This corresponds to an upper limit of R_M<6.1×10^-4 at the 90% C.L.

We report on a search for narrow-width particles decaying to a top and antitop quark pair. The data set used in the analysis corresponds to an integrated luminosity of 680  pb^-1 collected with the Collider Detector at Fermilab in run II. We present 95% confidence level upper limits on the cross section times branching ratio. Assuming a specific top-color-assisted technicolor production model, the leptophobic Z^′ with width Γ_Z^′=0.012M_Z^′, we exclude the mass range M_Z^′<725  GeV/c² at the 95% confidence level.