The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Hervé Partouche - One of the best experts on this subject based on the ideXlab platform.
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N=2→0 super no-Scale Models and moduli quantum stability
Elsevier, 2017Co-Authors: Costas Kounnas, Hervé PartoucheAbstract:We consider a class of heterotic N=2→0 super no-Scale Z2-orbifold Models. An appropriate stringy Scherk–Schwarz supersymmetry breaking induces tree level masses to all massless bosons of the twisted hypermultiplets and therefore stabilizes all twisted moduli. At high supersymmetry breaking Scale, the tachyons that occur in the N=4→0 parent theories are projected out, and no Hagedorn-like instability takes place in the N=2→0 Models (for small enough marginal deformations). At low supersymmetry breaking Scale, the stability of the untwisted moduli is studied at the quantum level by taking into account both untwisted and twisted contributions to the 1-loop effective potential. The latter depends on the specific branch of the gauge theory along which the background can be deformed. We derive its expression in terms of all classical marginal deformations in the pure Coulomb phase, and in some mixed Coulomb/Higgs phases. In this class of Models, the super no-Scale condition requires having at the massless level equal numbers of untwisted bosonic and twisted fermionic degrees of freedom. Finally, we show that N=1→0 super no-Scale Models are obtained by implementing a second Z2 orbifold twist on N=2→0 super no-Scale Z2-orbifold Models
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Super no-Scale Models in string theory
Nuclear Physics B, 2016Co-Authors: Costas Kounnas, Hervé PartoucheAbstract:We consider " super no-Scale Models " in the framework of the heterotic string, where the N = 4, 2, 1 → 0 spontaneous breaking of supersymmetry is induced by geometrical fluxes realizing a stringy Scherk– Schwarz perturbative mechanism. Classically, these backgrounds are characterized by a boson/fermion degeneracy at the massless level, even if supersymmetry is broken. At the 1-loop level, the vacuum energy is exponentially suppressed, provided the supersymmetry breaking Scale is small, m 3/2 M string. We show that the " super no-Scale string Models " under consideration are free of Hagedorn-like tachyonic singularities, even when the supersymmetry breaking Scale is large, m 3/2 M string. The vacuum energy decreases monotonically and converges exponentially to zero, when m 3/2 varies from M string to 0. We also show that all Wilson lines associated to asymptotically free gauge symmetries are dynamically stabilized by the 1-loop effective potential, while those corresponding to non-asymptotically free gauge groups lead to instabilities and condense. The Wilson lines of the conformal gauge symmetries remain massless. When stable , the stringy super no-Scale Models admit low energy effective actions, where decoupling gravity yields theories in flat spacetime, with softly broken supersymmetry.
Robert Vautard - One of the best experts on this subject based on the ideXlab platform.
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Evaluation and intercomparison of ozone and PM10 simulations by several chemistry transport Models over four European cities within the CityDelta project
Atmospheric environment, 2007Co-Authors: Robert Vautard, P. Builtjes, P. Thunis, K. Cuvelier, M. Bedogni, Bertrand Bessagnet, Cécile Honore, N. Moussiopoulos, Guido Pirovano, M. SchaapAbstract:The CityDelta project Cuvelier et al. [2006. CityDelta: a model intercomparison study to explore the impact of emission reductions in European cities in 2010. Atmospheric Environment] is designed to evaluate the air quality response of several emission abatement scenarios for 2010 at the Scale of the European continent, and specifically in the areas where most people live: the cities. Before evaluating this response, the model simulations in a control case must be evaluated against observations in order to understand their main strengths and weaknesses. In this article six different Models are used to simulate a full year (1999) of air quality pollutant concentrations over domains encompassing a large area around four major European cities: Berlin, Milan, Paris and Prague. Three Models are used both at large-Scale (typically 50 km) and small-Scale resolution (5 km). The intercomparison of the simulation results for ozone and particles smaller than 10 microns (PM10) leads to the following conclusions: (i) Models capture fairly well the mean, daily maxima and variability of ozone concentrations, as well as the time and intercity variability. However, a significant overestimation of ozone in city centres is found especially for large-Scale Models. (ii) PM10 simulation skill is generally poor, and large-Scale Models underestimate their mass. The difference between Milan (highly polluted) and the other cities is not reproduced. All Models have difficulties in capturing the observed seasonal variations. (iii) The fine Scale Models show higher PM10 and lower ozone concentrations in urban areas, which are closer to the observations than are the large-Scale Models.
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evaluation and intercomparison of ozone and pm10 simulations by several chemistry transport Models over four european cities within the citydelta project
Atmospheric Environment, 2007Co-Authors: Robert Vautard, P. Builtjes, P. Thunis, M. Bedogni, Bertrand Bessagnet, Cécile Honore, N. Moussiopoulos, C Cuvelier, Guido PirovanoAbstract:The CityDelta project Cuvelier et al. [2006. CityDelta: a model intercomparison study to explore the impact of emission reductions in European cities in 2010. Atmospheric Environment] is designed to evaluate the air quality response of several emission abatement scenarios for 2010 at the Scale of the European continent, and specifically in the areas where most people live: the cities. Before evaluating this response, the model simulations in a control case must be evaluated against observations in order to understand their main strengths and weaknesses. In this article six different Models are used to simulate a full year (1999) of air quality pollutant concentrations over domains encompassing a large area around four major European cities: Berlin, Milan, Paris and Prague. Three Models are used both at large-Scale (typically 50 km) and small-Scale resolution (5 km). The intercomparison of the simulation results for ozone and particles smaller than 10 microns (PM10) leads to the following conclusions:(i)Models capture fairly well the mean, daily maxima and variability of ozone concentrations, as well as the time and intercity variability. However, a significant overestimation of ozone in city centres is found especially for large-Scale Models.(ii)PM10 simulation skill is generally poor, and large-Scale Models underestimate their mass. The difference between Milan (highly polluted) and the other cities is not reproduced. All Models have difficulties in capturing the observed seasonal variations.(iii)The fine Scale Models show higher PM10 and lower ozone concentrations in urban areas, which are closer to the observations than are the large-Scale Models. © 2006 Elsevier Ltd. All rights reserved.
Jon Mcauliffe - One of the best experts on this subject based on the ideXlab platform.
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variational inference for large Scale Models of discrete choice
Journal of the American Statistical Association, 2010Co-Authors: Michael Braun, Jon McauliffeAbstract:Discrete choice Models are commonly used by applied statisticians in numerous fields, such as marketing, economics, finance, and operations research. When agents in discrete choice Models are assumed to have differing preferences, exact inference is often intractable. Markov chain Monte Carlo techniques make approximate inference possible, but the computational cost is prohibitive on the large datasets now becoming routinely available. Variational methods provide a deterministic alternative for approximation of the posterior distribution. We derive variational procedures for empirical Bayes and fully Bayesian inference in the mixed multinomial logit model of discrete choice. The algorithms require only that we solve a sequence of unconstrained optimization problems, which are shown to be convex. One version of the procedures relies on a new approximation to the variational objective function, based on the multivariate delta method. Extensive simulations, along with an analysis of real-world data, demonstr...
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variational inference for large Scale Models of discrete choice
arXiv: Methodology, 2007Co-Authors: Michael Braun, Jon McauliffeAbstract:Discrete choice Models are commonly used by applied statisticians in numerous fields, such as marketing, economics, finance, and operations research. When agents in discrete choice Models are assumed to have differing preferences, exact inference is often intractable. Markov chain Monte Carlo techniques make approximate inference possible, but the computational cost is prohibitive on the large data sets now becoming routinely available. Variational methods provide a deterministic alternative for approximation of the posterior distribution. We derive variational procedures for empirical Bayes and fully Bayesian inference in the mixed multinomial logit model of discrete choice. The algorithms require only that we solve a sequence of unconstrained optimization problems, which are shown to be convex. Extensive simulations demonstrate that variational methods achieve accuracy competitive with Markov chain Monte Carlo, at a small fraction of the computational cost. Thus, variational methods permit inferences on data sets that otherwise could not be analyzed without bias-inducing modifications to the underlying model.
Thomas J R Hughes - One of the best experts on this subject based on the ideXlab platform.
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multiScale phenomena green s functions the dirichlet to neumann formulation subgrid Scale Models bubbles and the origins of stabilized methods
Computer Methods in Applied Mechanics and Engineering, 1995Co-Authors: Thomas J R HughesAbstract:Abstract An approach is developed for deriving variational methods capable of representing multiScale phenomena. The ideas are first illustrated on the exterior problem for the Helmholtz equation. This leads to the well-known Dirichlet-to-Neumann formulation. Next, a class of subgrid Scale Models is developed and the relationships to ‘bubble function’ methods and stabilized methods are established. It is shown that both the latter methods are approximate subgrid Scale Models. The identification for stabilized methods leads to an analytical formula for τ, the ‘intrinsic time Scale’, whose origins have been a mystery heretofore.
Costas Kounnas - One of the best experts on this subject based on the ideXlab platform.
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N=2→0 super no-Scale Models and moduli quantum stability
Elsevier, 2017Co-Authors: Costas Kounnas, Hervé PartoucheAbstract:We consider a class of heterotic N=2→0 super no-Scale Z2-orbifold Models. An appropriate stringy Scherk–Schwarz supersymmetry breaking induces tree level masses to all massless bosons of the twisted hypermultiplets and therefore stabilizes all twisted moduli. At high supersymmetry breaking Scale, the tachyons that occur in the N=4→0 parent theories are projected out, and no Hagedorn-like instability takes place in the N=2→0 Models (for small enough marginal deformations). At low supersymmetry breaking Scale, the stability of the untwisted moduli is studied at the quantum level by taking into account both untwisted and twisted contributions to the 1-loop effective potential. The latter depends on the specific branch of the gauge theory along which the background can be deformed. We derive its expression in terms of all classical marginal deformations in the pure Coulomb phase, and in some mixed Coulomb/Higgs phases. In this class of Models, the super no-Scale condition requires having at the massless level equal numbers of untwisted bosonic and twisted fermionic degrees of freedom. Finally, we show that N=1→0 super no-Scale Models are obtained by implementing a second Z2 orbifold twist on N=2→0 super no-Scale Z2-orbifold Models
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Super no-Scale Models in string theory
Nuclear Physics B, 2016Co-Authors: Costas Kounnas, Hervé PartoucheAbstract:We consider " super no-Scale Models " in the framework of the heterotic string, where the N = 4, 2, 1 → 0 spontaneous breaking of supersymmetry is induced by geometrical fluxes realizing a stringy Scherk– Schwarz perturbative mechanism. Classically, these backgrounds are characterized by a boson/fermion degeneracy at the massless level, even if supersymmetry is broken. At the 1-loop level, the vacuum energy is exponentially suppressed, provided the supersymmetry breaking Scale is small, m 3/2 M string. We show that the " super no-Scale string Models " under consideration are free of Hagedorn-like tachyonic singularities, even when the supersymmetry breaking Scale is large, m 3/2 M string. The vacuum energy decreases monotonically and converges exponentially to zero, when m 3/2 varies from M string to 0. We also show that all Wilson lines associated to asymptotically free gauge symmetries are dynamically stabilized by the 1-loop effective potential, while those corresponding to non-asymptotically free gauge groups lead to instabilities and condense. The Wilson lines of the conformal gauge symmetries remain massless. When stable , the stringy super no-Scale Models admit low energy effective actions, where decoupling gravity yields theories in flat spacetime, with softly broken supersymmetry.
