4,023 research outputs found

    Locating transition states using double-ended classical trajectories

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    In this paper we present a method for locating transition states and higher-order saddles on potential energy surfaces using double-ended classical trajectories. We then apply this method to 7- and 8-atom Lennard-Jones clusters, finding one previously unreported transition state for the 7-atom cluster and two for the 8-atom cluster.Comment: Journal of Chemical Physics, 13 page

    Orbit determination support of the Ocean Topography Experiment (TOPEX)/Poseidon operational orbit

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    The Ocean Topography Experiment (TOPEX/Poseidon) mission is designed to determine the topography of the Earth's sea surface over a 3-year period, beginning shortly after launch in July 1992. TOPEX/Poseidon is a joint venture between the United States National Aeronautics and Space Administration (NASA) and the French Centre Nationale d'Etudes Spatiales. The Jet Propulsion Laboratory is NASA's TOPEX/Poseidon project center. The Tracking and Data Relay Satellite System (TDRSS) will nominally be used to support the day-to-day orbit determination aspects of the mission. Due to its extensive experience with TDRSS tracking data, the NASA Goddard Space Flight Center (GSFC) Flight Dynamics Facility (FDF) will receive and process TDRSS observational data. To fulfill the scientific goals of the mission, it is necessary to achieve and maintain a very precise orbit. The most stringent accuracy requirements are associated with planning and evaluating orbit maneuvers, which will place the spacecraft in its mission orbit and maintain the required ground track. To determine if the FDF can meet the TOPEX/Poseidon maneuver accuracy requirements, covariance analysis was undertaken with the Orbit Determination Error Analysis System (ODEAS). The covariance analysis addressed many aspects of TOPEX/Poseidon orbit determination, including arc length, force models, and other processing options. The most recent analysis has focused on determining the size of the geopotential field necessary to meet the maneuver support requirements. Analysis was undertaken with the full 50 x 50 Goddard Earth Model (GEM) T3 field as well as smaller representations of this model

    Flight dynamics facility operational orbit determination support for the ocean topography experiment

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    The Ocean Topography Experiment (TOPEX/POSEIDON) mission is designed to determine the topography of the Earth's sea surface across a 3 yr period, beginning with launch in June 1992. The Goddard Space Flight Center Dynamics Facility has the capability to operationally receive and process Tracking and Data Relay Satellite System (TDRSS) tracking data. Because these data will be used to support orbit determination (OD) aspects of the TOPEX mission, the Dynamics Facility was designated to perform TOPEX operational OD. The scientific data require stringent OD accuracy in navigating the TOPEX spacecraft. The OD accuracy requirements fall into two categories: (1) on orbit free flight; and (2) maneuver. The maneuver OD accuracy requirements are of two types; premaneuver planning and postmaneuver evaluation. Analysis using the Orbit Determination Error Analysis System (ODEAS) covariance software has shown that, during the first postlaunch mission phase of the TOPEX mission, some postmaneuver evaluation OD accuracy requirements cannot be met. ODEAS results also show that the most difficult requirements to meet are those that determine the change in the components of velocity for postmaneuver evaluation

    Phase changes in selected Lennard-Jones X_{13-n}Y_n clusters

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    Detailed studies of the thermodynamic properties of selected binary Lennard-Jones clusters of the type X_{13-n}Y_n (where n=1,2,3) are presented. The total energy, heat capacity and first derivative of the heat capacity as a function of temperature are calculated by using the classical and path integral Monte Carlo methods combined with the parallel tempering technique. A modification in the phase change phenomena from the presence of impurity atoms and quantum effects is investigated.Comment: 14 pages, 13 figures. submitted to J. Chem. Phy

    Limits on τ lepton-flavor violating decays into three charged leptons

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    A search for the neutrinoless, lepton-flavor violating decay of the τ lepton into three charged leptons has been performed using an integrated luminosity of 468  fb^(-1) collected with the BABAR detector at the PEP-II collider. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (1.8–3.3)×10^(-8) at 90% confidence level

    Theoretical Studies of the Structure and Dynamics of Metal/Hydrogen Systems: Diffusion and Path Integral Monte Carlo Investigations of Nickel and Palladium Clusters

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    Using both classical and quantum mechanical Monte Carlo methods, a number of properties are investigated for a single hydrogen atom adsorbed on palladium and nickel clusters. In particular, the geometries, the preferred binding sites, site specific hydrogen normal mode frequencies, and finite temperature effects in clusters from two to ten metal atoms are examined. Our studies indicate that hydrogen is localized in the present systems. The preferred hydrogen binding sites are found to be tetrahedral in clusters with five or fewer metal atoms and ctahedral for clusters of six to ten atoms. The exceptions to this rule are Ni9H and Pd9H for which the outside, threefold hollow and the inside tetrahedral sites are preferred, respectively. Hydrogen induced ‘‘reconstruction’’ of bare cluster geometries is seen in seven and ten-atom clusters

    Analysis of the D^+ → K^-π^+e^+ν_e decay channel

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    Using 347.5  fb^(-1) of data recorded by the BABAR detector at the PEP-II electron-positron collider, 244×10^3 signal events for the D^+ → K^-π^+e^+ν_e decay channel are analyzed. This decay mode is dominated by the K̅ ^*(892)^0 contribution. We determine the K̅ ^*(892)^0 parameters: m_(K^*(892)^0)=(895.4±0.2±0.2)  MeV/c^2, Γ_(K^*(892)^0)=(46.5±0.3±0.2)  MeV/c^2, and the Blatt-Weisskopf parameter r_(BW) =2.1±0.5±0.5  (GeV/c)^-1, where the first uncertainty comes from statistics and the second from systematic uncertainties. We also measure the parameters defining the corresponding hadronic form factors at q^2 = 0 (r_V = ^(V(0))/_(A1(0)) = 1.463 ± 0.017 ± 0.031, r_2 = _(A1(0)) ^(A2(0))= 0.801±0.020±0.020) and the value of the axial-vector pole mass parametrizing the q^2 variation of A_1 and A_2: m_A=(2.63±0.10±0.13)  GeV/c^2. The S-wave fraction is equal to (5.79±0.16±0.15)%. Other signal components correspond to fractions below 1%. Using the D^+ → K^-π^+π^+ channel as a normalization, we measure the D^+ semileptonic branching fraction: B(D^+ → K^-π^+e^+ν_e)=(4.00±0.03±0.04±0.09)×10^(-2), where the third uncertainty comes from external inputs. We then obtain the value of the hadronic form factor A_1 at q^2=0: A_1(0)=0.6200±0.0056±0.0065±0.0071. Fixing the P-wave parameters, we measure the phase of the S wave for several values of the Kπ mass. These results confirm those obtained with Kπ production at small momentum transfer in fixed target experiments

    Measurement of the γγ^*→η_c transition form factor

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    We study the reaction e^+e^-→e^+e^-η_c, η_c→K_SK^±π^∓ and obtain η_c mass and width values 2982.2±0.4±1.6  MeV/c^2 and 31.7±1.2±0.8  MeV, respectively. We find Γ(η_c→γγ)B(ηc→KK π)=0.374±0.009±0.031  keV, and measure the γγ^*→η_c transition form factor in the momentum transfer range from 2 to 50  GeV^2. The analysis is based on 469  fb^(-1) of integrated luminosity collected at PEP-II with the BABAR detector at e^+e^- center-of-mass energies near 10.6 GeV
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