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At least 1,027 records · Page 57

Gauge Fields, Membranes, and Subdeterminant Vector Models

We present a class of classically marginal N-vector models in d=4 and d=3 whose scalar potentials can be written as subdeterminants of symmetric matrices. The d=3 case can be thought of as a generalization of the scalar sector of the Bagger-Lambert-Gustavsson model. Using the Hubbard-Stratonovich transformation we calculate their effective potentials which exhibit intriguing large-N scaling behaviors. We comment on the possible relevance of our models to strings, membranes, and also to a class of novel spin systems that are based on ternary commutation relations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Interacting Hofstadter Spectrum of Atoms in an Artificial Gauge Field

Motivated by experimental advances in the synthesis of gauge potentials for ultracold atoms, we consider the superfluid phase of interacting bosons on a square lattice in the presence of a magnetic field. We show that superfluid order implies spatial symmetry breaking, and predict clear signatures of many-body effects in time-of-flight measurements. By developing a Bogoliubov expansion based on the exact Hofstadter spectrum, we find the dispersion of the quasiparticle modes within the superfluid phase, and describe the consequences for Bragg spectroscopy measurements. The theory also provides an estimate of the critical interaction strength at the transition to the Mott insulator phase.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Curvatons in Warped Throats

We present a curvaton model from type IIB string theory compactified on a warped throat with approximate isometries. Considering an (anti-)D3-brane sitting at the throat tip as a prototype standard model brane, we show that the brane's position in the isometry directions can play the role of curvatons. The basic picture is that the fluctuations of the (anti-)D3-brane in the angular isometry directions during inflation eventually turns into the primordial curvature perturbations, and subsequently the brane's oscillation excites other open string modes on the brane and reheat the universe. We find in the explicit case of the KS throat that a wide range of parameters allows a consistent curvaton scenario. It is also shown that the oscillations of branes at throat tips are capable of producing large non-Gaussianity, either through curvature or isocurvature perturbations. Since such setups naturally arise in warped (multi-)throat compactifications and are constrained by observational data, the model can provide tests for compactification scenarios. This work gives an explicit example of string theory providing light fields for generating curvature perturbations. Such mechanisms free the inflaton from being responsible for the perturbations, thus open up new possibilities for inflation models. The discussions in this paper are based on [1].

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Removing numerical pathologies in a turbulence parameterization through convergence testing

Discretized numerical models of the atmosphere are usually intended to faithfully represent an underlying set of continuous equations, but this necessary condition is violated sometimes by subtle pathologies that have crept into the discretized equations. Such pathologies can introduce undesirable artifacts, such as sawtooth noise, into the model solutions. The presence of these pathologies can be detected by numerical convergence testing. This study employs convergence testing to verify the discretization of the Cloud Layers Unified By Binormals (CLUBB) model of clouds and turbulence. That convergence testing identifies two aspects of CLUBB's equation set that contribute to undesirable noise in the solutions. First, numerical limiters (i.e. clipping) used by CLUBB introduce discontinuities or slope discontinuities in model fields. Second, this noise can be amplified by an advective term in CLUBB's background diffusion. Smoothing the limiters and removing the advective component of the background diffusion reduces the noise and restores the expected first-order convergence in CLUBB's solutions. These model reformulations improve the results at coarser, near-operational grid spacing and time step in cumulus cloud and dry turbulence tests. In addition, convergence testing is proved to be a valuable tool for detecting pathologies, including unintended discontinuities and grid dependence, in the model equation set.

Mathematics and Computing, Geosciences↗

Removing Numerical Pathologies in a Turbulence Parameterization Through Convergence Testing

Abstract Discretized numerical models of the atmosphere are usually intended to faithfully represent an underlying set of continuous equations, but this necessary condition is violated sometimes by subtle pathologies that have crept into the discretized equations. Such pathologies can introduce undesirable artifacts, such as sawtooth noise, into the model solutions. The presence of these pathologies can be detected by numerical convergence testing. This study employs convergence testing to verify the discretization of the Cloud Layers Unified By Binormals (CLUBB) model of clouds and turbulence. That convergence testing identifies two aspects of CLUBB's equation set that contribute to undesirable noise in the solutions. First, numerical limiters (i.e., clipping) used by CLUBB introduce discontinuities or slope discontinuities in model fields. Second, nonlinear artificial diffusion employed for improving numerical stability can introduce unintended small‐scale features into the solution of the model equations. Smoothing the limiters and using linear artificial diffusion reduces the noise and restores the expected first‐order convergence in CLUBB's solutions. These model reformulations enhance our confidence in the trustworthiness of solutions from CLUBB by eliminating the unphysical oscillations in high‐resolution simulations. The improvements in the results at coarser, near‐operational grid spacing and timestep are also seen in cumulus cloud and dry turbulence tests. In addition, convergence testing is proven to be a valuable tool for detecting pathologies, including unintended discontinuities and grid dependence, in the model equation set.

58 GEOSCIENCES↗

Edible oil structures at low and intermediate concentrations. II. Ultra-small angle X-ray scattering of in situ tristearin solids in triolein

Ultra-small angle X-ray scattering has been used for the first time to elucidate, in situ, the aggregation structure of a model edible oil system. The three-dimensional nano- to micro-structure of tristearin solid particles in triolein solvent was investigated using 5, 10, 15, and 20% solids. Three different sample preparation procedures were investigated: two slow cooling rates of 0.5°/min, case 1 (22 days of storage at room temperature) and case 2 (no storage), and one fast cooling of 30°/min, case 3 (no storage). The length scale investigated, by using the Bonse-Hart camera at beamline ID-15D at the Advanced Photon Source, Argonne National Laboratory, covered the range from 300 Å to 10 μm. The unified fit and the Guinier-Porod models in the Irena software were used to fit the data. The former was used to fit 3 structural levels. Level 1 structures showed that the primary scatterers were essentially 2-dimensional objects for the three cases. The scatterers possessed lateral dimensions between 1000 and 4300 Å. This is consistent with the sizes of crystalline nanoplatelets present which were observed using cryo-TEM. Level 2 structures were aggregates possessing radii of gyration, R{sub g2} between 1800 Å and 12000 Å and fractal dimensions of either D{sub 2}=1 for case 3 or 1.8≤D{sub 2}≤2.1 for case 1 and case 2. D{sub 2} = 1 is consistent with unaggregated 1-dimensional objects. 1.8 ≤ D{sub 2} ≤ 2.1 is consistent with these 1-dimensional objects (below) forming structures characteristic of diffusion or reaction limited cluster-cluster aggregation. Level 3 structures showed that the spatial distribution of the level 2 structures was uniform, on the average, for case 1, with fractal dimension D{sub 3}≈3 while for case 2 and case 3 the fractal dimension was D{sub 3}≈2.2, which suggested that the large-scale distribution had not come to equilibrium. The Guinier-Porod model showed that the structures giving rise to the aggregates with a fractal dimension given by D{sub 2} in the unified fit level 2 model were cylinders described by the parameter s≈1 in the Guinier-Porod model. The size of the base of these cylinders was in agreement with the cryo-TEM observations as well as with the results of the level 1 unified fit model. By estimating the size of the nanoplatelets and understanding the structures formed via their aggregation, it will be possible to engineer novel lipids systems that embody desired functional characteristics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Macroscopic modeling of gas permeability in hierarchical micro/nanoporous media: A unified characterization of rarefaction using Klinkenberg theory and equivalent diameter

Estimating gas transport through a hierarchical micro/nanoporous system is challenging due to non-equilibrium gas dynamics. The primary difficulty lies in determining the rarefaction level, because identifying a representative flow dimension in a complex porous system with multiple pore scales is not straightforward. Our study performed a pore-level analysis for gas permeability in dual-scale porous media with varying porosity, throat size, and secondary pore size under different rarefaction conditions. We found that secondary porosity negatively affects permeability due to increased friction forces, with this influence growing as the secondary pore size and porosity increase until the secondary pore becomes comparable to the throat. However, rarefaction reduces the effects of secondary pores due to boundary slip. Traditional Knudsen number (Kn) calculations based on Darcy-defined height failed to accurately describe the rarefaction effects on gas permeability. Instead, we introduced an equivalent diameter to calculate the Kn, which provided an accurate normalization of apparent gas permeability independent of pore geometry. Furthermore, the extended Kozeny–Carman–Klinkenberg model developed in our previous study successfully yielded a macroscopic model for apparent gas permeability in hierarchical micro/nanoporous systems as a function of the traditional Darcy height and porosity.

04 OIL SHALES AND TAR SANDS↗

Higgs boson hadronic branching ratios at the ILC

We present a study of the Higgs boson decay branching ratios to bb, cc, and gluons, one of the cornerstones of the physics program at the International Linear Collider. A standard model Higgs boson of 120 GeV mass, produced in the Higgs-strahlung process at {radical}(s)=250 GeV, was investigated using the full detector simulation and reconstruction procedures. The analysis was performed in the framework of the Silicon Detector concept with full account of inclusive standard model backgrounds. The selected decay modes contained two heavy flavor jets in the final state and required excellent flavor tagging through precise reconstruction of interaction and decay vertices in the detector. A new signal discrimination technique using correlations of neural network outputs was used to determine the branching ratios and estimate their uncertainties, 4.8%, 8.4%, and 12.2% for bb, cc, and gluons, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

EXTREME ACTIVE MOLECULAR JETS IN L1448C

The protostellar jet driven by L1448C was observed in the SiO J = 8-7 and CO J = 3-2 lines, and 350 GHz dust continuum at {approx}1'' resolution with the Submillimeter Array. A narrow jet from the northern source L1448C(N) was observed in the SiO and the high-velocity CO. The jet consists of a chain of emission knots with an inter-knot spacing of {approx}2'' (500 AU) and a semi-periodic velocity variation. These knots are likely to be the internal bow shocks in the jet beam that were formed due to the periodic variation of the ejection velocity with a period of {approx}15-20 yr. The innermost pairs of knots, which are significant in the SiO map but barely seen in the CO, are located at {approx}1'' (250 AU) from the central source, L1448C(N). Since the dynamical timescale for the innermost pair is only {approx}10 yr, SiO may have been formed in the protostellar wind through the gas-phase reaction, or formed on the dust grain and directly released into the gas phase by means of shocks. It is found that the jet is extremely active with a mechanical luminosity of {approx}7 L{sub sun}, which is comparable to the bolometric luminosity of the central source (7.5 L{sub sun}). The mass accretion rate onto the protostar derived from the mass-loss rate is {approx}10{sup -5} M{sub sun} yr{sup -1}. Such a high mass accretion rate suggests that the mass and the age of the central star are 0.03-0.09 M{sub sun} and (4-12)x10{sup 3} yr, respectively, implying that the central star is in the very early stage of protostellar evolution. The low-velocity CO emission delineates two V-shaped shells with a common apex at L1448C(N). The kinematics of these shells are reproduced by the model of a wide-opening angle wind. The co-existence of the highly collimated jets and the wide-opening angle shells can be explained by the 'unified X-wind model' in which highly collimated jet components correspond to the on-axis density enhancement of the wide-opening angle wind. The CO J = 3-2 map also revealed the second outflow driven by the southern source L1448C(S) located at {approx}8.''3 (2000 AU) from L1448C(N). Although L1448C(S) is brighter than L1448C(N) in the mid-IR bands, the momentum flux of the outflow from L1448C(S) is 2 or 3 orders of magnitude smaller than that of the L1448C(N) outflow. It is likely that the evolution of L1448C(S) has been strongly affected by the powerful outflow from L1448C(N).

79 ASTRONOMY AND ASTROPHYSICS↗

Credible practice of modeling and simulation in healthcare: ten rules from a multidisciplinary perspective

The complexities of modern biomedicine are rapidly increasing. Thus, modeling and simulation have become increasingly important as a strategy to understand and predict the trajectory of pathophysiology, disease genesis, and disease spread in support of clinical and policy decisions. In such cases, inappropriate or ill-placed trust in the model and simulation outcomes may result in negative outcomes, and hence illustrate the need to formalize the execution and communication of modeling and simulation practices. Although verification and validation have been generally accepted as significant components of a model’s credibility, they cannot be assumed to equate to a holistic credible practice, which includes activities that can impact comprehension and in-depth examination inherent in the devel-opment and reuse of the models. For the past several years, the Committee on Credible Practice of Modeling and Simulation in Healthcare, an interdisciplinary group seeded from a U.S. interagency initiative, has worked to codify best practices. Here, we provide Ten Rules for credible practice of modeling and simulation in healthcare developed from a comparative analysis by the Committee’s multidisciplinary membership, followed by a large stakeholder com-munity survey. These rules establish a unified conceptual framework for modeling and simulation design, implementation, evaluation, dissemination and usage across the modeling and simulation life-cycle. While biomedical science and clinical care domains have somewhat different requirements and expectations for credible practice, our study converged on rules that would be useful across a broad swath of model types. In brief, the rules are: (1) Define context clearly. (2) Use contextually appropriate data. (3) Evaluate within context. (4) List limitations explicitly. (5) Use version control. (6) Document appropriately. (7) Disseminate broadly. (8) Get independent reviews. (9) Test competing imple-mentations. (10) Conform to standards. Although some of these are common sense guidelines, we have found that many are often missed or misconstrued, even by seasoned practitioners. Computational models are already widely used in basic science to generate new biomedical knowledge. As they penetrate clinical care and healthcare policy, contributing to personalized and precision medicine, clinical safety will require established guidelines for the credible practice of modeling and simulation in healthcare.

Credibility↗

Top quark forward-backward asymmetry from the 3-3-1 model

The forward-backward asymmetry A{sub FB} in top quark pair production, measured at the Tevatron, is probably related to the contribution of new particles. The Tevatron result is more than a 2{sigma} deviation from the standard model prediction and motivates the application of alternative models introducing new states. However, as the standard model predictions for the total cross section {sigma}{sub tt} and invariant mass distribution M{sub tt} for this process are in good agreement with experiments, any alternative model must reproduce these predictions. These models can be placed into two categories: One introduces the s-channel exchange of new vector bosons with chiral couplings to the light quarks and to the top quark, and another relies on the t-channel exchange of particles with large flavor-violating couplings in the quark sector. In this work, we employ a model which introduces both s- and t-channel nonstandard contributions for the top quark pair production in proton-antiproton collisions. We use the minimal version of the SU(3){sub C} x SU(3){sub L} x U(1){sub X} model (3-3-1 model) that predicts the existence of a new neutral gauge boson, called Z{sup '}. This gauge boson has both flavor-changing couplings to up and top quarks and chiral coupling to the light quarks and to the top quark. This very peculiar model coupling can correct the A{sub FB} for top quark pair production for two ranges of Z{sup '} mass while leading to a cross section and invariant mass distribution quite similar to the standard model ones. This result reinforces the role of the 3-3-1 model for any new physics effect.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Poker face of inelastic dark matter: Prospects at upcoming direct detection experiments

The XENON100 and CRESST experiments will directly test the inelastic dark matter explanation for DAMA's 8.9{sigma} anomaly. This article discusses how predictions for direct detection experiments depend on uncertainties in quenching factor measurements, the dark matter interaction with the standard model, and the halo velocity distribution. When these uncertainties are accounted for, an order of magnitude variation is found in the number of expected events at CRESST and XENON100.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Forward-backward asymmetry of top quark pair production

We adopt a Markov chain Monte Carlo method to examine various new physics models which can generate the forward-backward asymmetry in top quark pair production observed at the Tevatron by the CDF Collaboration. We study the following new physics models: (1) exotic gluon G{sup '}, (2) extra Z{sup '} boson with flavor-conserving interaction, (3) extra Z{sup '} with flavor-violating u-t-Z{sup '} interaction, (4) extra W{sup '} with flavor-violating d-t-W{sup '} interaction, and (5) extra scalars S and S{sup {+-}}with flavor-violating u-t-S and d-t-S{sup {+-}}interactions. After combining the forward-backward asymmetry with the measurement of the top pair production cross section and the tt invariant mass distribution at the Tevatron, we find that an axial vector exotic gluon G{sup '} of mass about 1 TeV or 2 TeV or a W{sup '} of mass about 2TeV offer an improvement over the standard model. The other models considered do not fit the data significantly better than the standard model. We also emphasize a few points that have been long ignored in the literature for new physics searches: (1) heavy resonance width effects, (2) renormalization scale dependence, and (3) next-to-leading order corrections to the tt invariant mass spectrum. We argue that these three effects are crucial to test or exclude new physics effects in the top quark pair asymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Comparison of Micromechanics Theories for Modeling Discontinuous Long Fiber Reinforced Composites

Two micromechanics theories, the high fidelity generalized method of cells and Carrera Unified Formulation, are used to model a repeating unit cell of a discrete long fiber composite. The models are used to predict the effective properties of a single fiber with varying fiber-to-fiber end gap lengths, as well as the local stresses in the fiber and matrix. Both models produced similar results and demonstrated that they could capture the complex local load transfer due to the shear lag effect.

discrete long fiber composites↗

Theoretical Foundation for the Index Theorem on the Lattice with Staggered Fermions

A way to identify the would-be zero modes of staggered lattice fermions away from the continuum limit is presented. Our approach also identifies the chiralities of these modes, and their index is seen to be determined by gauge field topology in accordance with the index theorem. The key idea is to consider the spectral flow of a certain Hermitian version of the staggered Dirac operator. The staggered fermion index thus obtained can be used as a new way to assign the topological charge of lattice gauge fields. In a numerical study in U(1) backgrounds in two dimensions it is found to perform as well as the Wilson index while being computationally more efficient. It can also be expressed as the index of an overlap Dirac operator with a new staggered fermion kernel.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Baryogenesis by B-L generation due to superheavy particle decay

We have shown that the B-L generation due to the decay of the thermally produced superheavy fields can explain the Baryon asymmetry in the Universe if the superheavy fields are heavier than 10{sup 13-14} GeV. Note that, although the superheavy fields have nonvanishing charges under the standard model gauge interactions, the thermally produced baryon asymmetry is sizable. The B-L-violating effective operators induced by integrating the superheavy fields have dimension 7, while the operator in the famous leptogenesis has dimension 5. Therefore, the constraints from the nucleon stability can be easily satisfied.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

A unified finite strain gradient-enhanced micropolar continuum approach for modeling quasi-brittle failure of cohesive-frictional materials

Here, in this work, a novel framework for modeling quasi-brittle crack propagation and shear band dominated failure of cohesive-frictional materials like concrete, mortar, rock, tough ceramics, energetic materials, but also granular materials like sands or powders in terms of a unified continuum approach is proposed. It is based on a combination of the gradient-enhanced continuum with gradients of internal variables for representing quasi-brittle cracking, and the micropolar continuum, accounting for the deformation of the microstructure. For developing the gradient-enhanced micropolar framework, the set of balance equations and the kinematic relations are derived, and the constitutive relations are established in a general manner. The framework is formulated in a geometrically exact setting, based on the thermodynamically sound theory of hyperelasto-plasticity, and the numerical implementation by means of the finite element method is discussed. For assessing the approach, realizations of this new approach in terms of constitutive models for particular materials are developed. They are applied to numerical benchmark examples, investigating various loading conditions, and the obtained results are validated by means of a comparison with experiments from the literature.

36 MATERIALS SCIENCE↗