Search Results (1,732 total)

This research focuses on the electrokinetic motion of fullerenes suspended in liquid crystal host medium, which are investigated in the homogeneously aligned nematic liquid crystal cells driven by in-plane field. We investigated the effect of electrophoretic and dielectrophoretic forces and related parameters of the colloidal fullerenes in liquid crystals. The electrophoretic mobility, zeta potential, and critical voltage have been evaluated. Fullerenes suspended in liquid crystal medium migrated toward the positive electrode, but were pulled back in the opposite direction when the polarity was reversed especially at low frequency range (<5 Hz). At higher electric field and higher frequency ranges, the net displacement of fullerenes has been observed. We demonstrate that the dielectrophoretic force dominated the motion in the colloidal fullerenes by a proper analysis of different electrophoretic parameters. In addition, the electrodynamics of fullerenes was explained by applying the theory of the dielectrophoresis and Schwarz’s formula. We propose a model to estimate the density of fullerenes suspended in liquid crystal medium.

We report measurements of the exclusive cross section for e⁺e^-→D⁰D^*-π⁺ as a function of center-of-mass energy from the D⁰D^*-π⁺ threshold to 5.2 GeV with initial-state radiation. No evidence is found for Y(4260)→D⁰D^*-π⁺ decays. The analysis is based on a data sample collected with the Belle detector at or near a center-of-mass energy of 10.58 GeV with an integrated luminosity of 695  fb^-1 at the KEKB asymmetric-energy e⁺e^- collider.

Observation of a tribo-induced transition from solid to liquidlike behavior is reported for a scanning tunneling microscope tip in sliding contact with an indium electrode of a quartz crystal microbalance (QCM). In particular, at a sufficiently high asperity sliding speed (about 1  m/s) and/or sample temperature, a change in the contact mechanics is observed that is consistent with melting in terms of both the QCM response and an energy analysis. The results confirm that the surface, rather than bulk, melting point temperature is the more relevant quantity for tribological considerations.

We present the results of a search for Higgs bosons predicted in two-Higgs-doublet models, in the case where the Higgs bosons decay to tau lepton pairs, using 1.8  fb^-1 of integrated luminosity of pp̅ collisions recorded by the CDF II experiment at the Fermilab Tevatron. Studying the mass distribution in events where one or both tau leptons decay leptonically, no evidence for a Higgs boson signal is observed. The result is used to infer exclusion limits in the two-dimensional space of tan⁡β versus m_A (the ratio of the vacuum expectation values of the two Higgs doublets and the mass of the pseudoscalar boson, respectively).

A combined mass and particle identification fit is used to make the first observation of the decay B̅ _s⁰→D_s^±K^∓ and measure the branching fraction of B̅ _s⁰→D_s^±K^∓ relative to B̅ _s⁰→D_s⁺π^-. This analysis uses 1.2  fb^-1 integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV collected with the CDF II detector at the Fermilab Tevatron collider. We observe a B̅ _s⁰→D_s^±K^∓ signal with a statistical significance of 8.1σ and measure B(B̅ _s⁰→D_s^±K^∓)/B(B̅ _s⁰→D_s⁺π^-)=0.097±0.018(stat)±0.009(syst).

The microscopic origin of the Landé g-factor in two ferromagnetic/nonmagnetic (FM/NM) bilayer systems-Co/Cu and Ni/Pd-has been investigated using x-ray magnetic circular dichroism, resonant magnetic reflectivity, and band calculations. The FM/NM bilayer represents the building block of any complete spin-transfer structure (FM1/NM/FM2). The valence electronic structure is profoundly altered over a finite length across the FM/NM interface. A considerable charge transfer takes place from the NM to the FM material. This results in an enhancement of the orbital-to-spin magnetic moment ratio in the FM layer and an induced magnetic polarization in the NM layer. Both effects turn out to be crucial for a correct understanding of the g-factor in spin-transfer systems.

A bent-shaped molecule exhibiting the mesophase sequence of B6-B1 was studied to understand the structural characteristics of the B6 phase compared with those of the B1 phase. A well-oriented sample was prepared in a magnetic field and examined by wide-angle x-ray diffraction measurements through the B6-B1 phase transition. The B6 phase has been considered to show only the (002) reflection, but this B6 phase showed broad scattering just inside the (002) reflection. The broad scattering has an intensity maximum at a very similar position to that of the (101) reflection in B1 and evolves into the well-defined (101) reflection on cooling into B1. Thus, B6 has a similar frustrated structure to B1, but the size of the antidomain in B6 may be dynamically distributed while B1 possesses an antidomain of definite size.

We study B→K^(*)ℓ⁺ℓ^- decays (ℓ=e, μ) based on a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. We report the differential branching fraction, isospin asymmetry, K^* polarization, and the forward-backward asymmetry (A_FB) as functions of q²=M_ℓℓ²c². The fitted A_FB spectrum exceeds the standard model expectation by 2.7 standard deviations. The measured branching fractions are B(B→K^*ℓ⁺ℓ^-)=(10.7_-1.0^+1.1±0.9)×10^-7 and B(B→Kℓ⁺ℓ^-)=(4.8_-0.4^+0.5±0.3)×10^-7, where the first errors are statistical and the second are systematic, with the muon to electron ratios R_K^*=0.83±0.17±0.08 and R_K=1.03±0.19±0.06.

Spin-polarized neutron-reflectivity measurements have been performed on superconducting Pb films. For fields applied in the plane of the film we are able to determine the magnetic field profile across the thickness of the film in the superconducting state. This allows the direct observation of distinct mesoscopic ground states in this geometry, from Meissner expulsion to a double row of vortices, which occur as a function of film thickness and external magnetic field. The data can be compared directly with solutions of the time-dependent Ginzburg-Landau equations, where we demonstrate good agreement between data and simulation. This provides a powerful demonstration of this widely applicable experimental technique to study mesoscopic ground states and a useful validation of this theoretical approach.

We present a search for associated production of the standard model Higgs boson and a Z boson where the Z boson decays to two leptons and the Higgs decays to a pair of b quarks in pp̅ collisions at the Fermilab Tevatron. We use event probabilities based on standard model matrix elements to construct a likelihood function of the Higgs content of the data sample. In a CDF data sample corresponding to an integrated luminosity of 2.7  fb^-1 we see no evidence of a Higgs boson with a mass between 100  GeV/c² and 150  GeV/c². We set 95% confidence level upper limits on the cross section for ZH production as a function of the Higgs boson mass m_H; the limit is 8.2 times the standard model prediction at m_H=115  GeV/c².

We report the observation of the bottom, doubly-strange baryon Ω_b^- through the decay chain Ω_b^-→J/ψΩ^-, where J/ψ→μ⁺μ^-, Ω^-→ΛK^-, and Λ→pπ^-, using 4.2  fb^-1 of data from pp̅ collisions at sqrt[s]=1.96  TeV, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is 4.0×10^-8, or 5.5 Gaussian standard deviations. The Ω_b^- mass is measured to be 6054.4±6.8(stat)±0.9(syst)  MeV/c². The lifetime of the Ω_b^- baryon is measured to be 1.13_-0.40^+0.53(stat)±0.02(syst)  ps. In addition, for the Ξ_b^- baryon we measure a mass of 5790.9±2.6(stat)±0.8(syst)  MeV/c² and a lifetime of 1.56_-0.25^+0.27(stat)±0.02(syst)  ps. Under the assumption that the Ξ_b^- and Ω_b^- are produced with similar kinematic distributions to the Λ_b⁰ baryon, we find σ(Ξ_b^-)B(Ξ_b^-→J/ψΞ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.167_-0.025^+0.037(stat)±0.012(syst) and σ(Ω_b^-)B(Ω_b^-→J/ψΩ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.045_-0.012^+0.017(stat)±  0.004(syst) for baryons produced with transverse momentum in the range of 6–20  GeV/c.

We present an analysis of the mass of the X(3872) reconstructed via its decay to J/ψπ⁺π^- using 2.4  fb^-1 of integrated luminosity from pp̅ collisions at sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. The possible existence of two nearby mass states is investigated. Within the limits of our experimental resolution the data are consistent with a single state, and having no evidence for two states we set upper limits on the mass difference between two hypothetical states for different assumed ratios of contributions to the observed peak. For equal contributions, the 95% confidence level upper limit on the mass difference is 3.6  MeV/c². Under the single-state model the X(3872) mass is measured to be 3871.61±0.16(stat)±0.19(syst)  MeV/c², which is the most precise determination to date.

Dür [Phys. Rev. Lett. 87, 230402 (2001)] constructed N-qubit bound entangled states which violate a Bell inequality for N≥8, and his result was recently improved by showing that there exists an N-qubit bound entangled state violating the same Bell inequality if and only if N≥6 [Phys. Rev. A 79, 032309 (2009)]. On the other hand, it has been also shown that the states which Dür considered violate Bell inequalities different from the inequality for N≥6. In this paper, by employing different forms of Bell inequalities, in particular, a specific form of Bell inequalities with M settings of the measuring apparatus for sufficiently large M, we prove that there exists an N-qubit bound entangled state violating the M-setting Bell inequality if and only if N≥4.

We report on a search for the process pp̅ →γ+W/Z with W/Z→qq̅ in events containing two jets and a photon at the center-of-mass energy sqrt[s]=1.96  TeV, using 184  pb^-1 of data collected by the CDF II detector. A neural network event selection has been developed to optimize the rejection of the large QCD production background; it is shown that this method gives a significant improvement in both signal-to-noise ratio and signal sensitivity, as compared with an event selection based on conventional cuts. An upper limit is presented for the γ+W/Z production cross section with the W and Z decaying hadronically.

Hall effect of two-dimensional holes that are spin-polarized by a strong parallel magnetic field was explored with a small tilt angle. The Hall resistivity increases nonlinearly with the magnetic field and exhibits negative corrections. The anomalous negative corrections increase with the perpendicular magnetization of the two-dimensional hole system. We attribute this to the anomalous Hall effect of spin-polarized carriers in a nonmagnetic system. The anomalous corrections to the conductivity exhibit nonmonotonic dependence on the magnetic field.

We report a measurement of the exclusive B⁺ meson decay to the D_s^(*)-K⁺π⁺ final state using 657×10⁶BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We use D_s^*-→D_s^-γ and the D_s^-→ϕπ^-, K̅ ^*(892)⁰K^- and K_S⁰K^- decay modes for D_s^(*) reconstruction and measure the following branching fractions: B(B⁺→D_s^-K⁺π⁺)=(1.71_-0.07^+0.08(stat)_-0.20^+0.20(syst)±0.15(B_int))×10^-4 and B(B⁺→D_s^*-K⁺π⁺)=(1.31_-0.12^+0.13(stat)_-0.25^+0.25(syst)±0.12(B_int))×10^-4. The uncertainties are due to statistics, experimental systematic errors, and uncertainties of intermediate branching fractions, respectively.

We use first-principles density-functional theory calculations to evaluate the orientation-dependent stability of small, neutral self-interstitial clusters (I_n, n≤4) in crystalline Si across a range of uniform strain conditions (−4≤ε≤4%) in both uniaxial and biaxial strain fields on Si(100). Comprehending the behavior of these small clusters under strain is important in extending our understanding of the evolutionary cycle of interstitial defects during the ion implantation and annealing processes that occur during semiconductor manufacturing. Our calculation results suggest that strain of sufficient magnitude can contribute to significant ground-state structural distortion and even generation of different cluster configurations. Our study also indicates that the relative stability change per unit change in applied strain is greater in the biaxial case than the uniaxial case for interstitial clusters. We provide localized strain-distribution profiles and modification of bulk Si density of states to characterize the extent to which interstitial clusters modulate crystalline Si structure.

We present a signature-based search for the anomalous production of events containing a photon, two jets, of which at least one is identified as originating from a b quark, and missing transverse energy (E̸_T). The search uses data corresponding to 2.0  fb^-1 of integrated luminosity from pp̅ collisions at a center-of-mass energy of sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. From 6.697 47×10⁶ events with a photon candidate with transverse energy E_T>25  GeV, we find 617 events with E̸_T>25  GeV and two or more jets with E_T>15  GeV, at least one identified as originating from a b quark, versus an expectation of 607±113 events. Increasing the requirement on E̸_T to 50 GeV, we find 28 events versus an expectation of 30±11 events. We find no indications of non-standard-model phenomena.

We report a kinematically complete experiment of carbon 1s photoionization of CO₂ including Auger decay and fragmentation. By measuring in coincidence of CO₂ C(1s) photoelectrons and ion fragments using synchrotron light at several energies above the C(1s) threshold, we determine photoelectron angular distributions as well as Auger-electron angular distributions with full solid angle in the molecular fixed frame. We confirm recent unexpected results showing an asymmetry of the photoelectron angular distribution along the molecular axis after ionization of the C(1s) orbital. Our high statistics and high resolution measurement unveils asymmetric features in the photoelectron angular distribution which change as a function of the kinetic energy release. This finding provides strong evidence that varying C–O bondlengths are the main cause for these asymmetries. The Auger-electron angular distributions do not show strong correlation with the photoelectrons.

We have used the Collider Detector at Fermilab (CDF-II) to search for the flavor-changing neutral-current (FCNC) top-quark decay t→Zc using a technique employing ratios of W and Z production, measured in pp̅ data corresponding to an integrated luminosity of 1.52  fb^-1. The analysis uses a comparison of two decay chains, pp̅ →tt̅ →WbWb→ℓνbjjb and pp̅ →tt̅ →ZcWb→ℓℓcjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multiyear data set using another ratio of W and Z production, in this case the observed ratio of inclusive production of W to Z bosons. To improve the discrimination against standard model backgrounds to top-quark decays, we calculate the top-quark mass for each event with two leptons and four jets assuming it is a tt̅ event with one of the top quarks decaying to Zc. For additional background discrimination we require at least one jet to be identified as originating from a b quark. No significant signal is found and we set an upper limit on the FCNC branching ratio Br(t→Zc) using a likelihood constructed from the ℓℓcjjb top-quark mass distribution and the number of ℓνbjjb events. Limits are set as a function of the helicity of the Z boson produced in the FCNC decay. For 100% longitudinally-polarized Z bosons we find limits of 8.3% and 9.3% (95% C.L.) depending on the assumptions regarding the theoretical top-quark pair production cross section.

We present the first measurement of the mass of the top quark in a sample of tt̅ →ℓν̅ bb̅ qq̅ events (where ℓ=e,μ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon b tagging). The pp̅ collision data used correspond to an integrated luminosity of 2  fb^-1 and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the W boson of the t→Wb decay and the muon from a semileptonic b decay. We fit template histograms, derived from simulation of tt̅ events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180.5±12.0(stat)±3.6(syst)  GeV/c², consistent with the current world average value.

We report on the first direct search for charged Higgs bosons decaying into cs̅ in tt̅ events produced by pp̅ collisions at sqrt[s]=1.96  TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2  fb^-1 collected by the CDF II detector at Fermilab and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of tt̅ candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at B(t→H⁺b)< 0.1 to 0.3 for charged Higgs boson masses of 60 to 150  GeV/c² assuming B(H⁺→cs̅ )=1.0. The upper limits on B(t→H⁺b) are also used as model-independent limits on the decay branching ratio of top quarks to generic scalar charged bosons beyond the standard model.

We study nucleon-to-delta electromagnetic transition form factors and relations between them within the framework of the holographic dual model of QCD proposed by Sakai and Sugimoto. In this setup, baryons appear as topological solitons of the five-dimensional holographic gauge theory that describes a tower of mesons and their interactions. We find a relativistic extension of the nucleon-delta-vector meson interaction vertices and use these to calculate transition form factors from holographic QCD. We observe that at low momentum transfer, magnetic dipole, electric and Coulomb quadrupole form factors, and their ratios follow the patterns expected in the large N_c limit. Our results at this approximation are in reasonable agreement with experiment.

We present a search for a standard model Higgs boson produced in association with a W boson using 2.7  fb^-1 of integrated luminosity of pp̅ collision data taken at sqrt[s]=1.96  TeV. Limits on the Higgs boson production rate are obtained for masses between 100 and 150  GeV/c². Through the use of multivariate techniques, the analysis achieves an observed (expected) 95% confidence level upper limit of 5.6 (4.8) times the theoretically expected production cross section for a standard model Higgs boson with a mass of 115  GeV/c².