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Mariano I. Loza, Argentina

Peralta-Ramos J.,Sao Paulo State University | Calzetta E.,FCEyN UBA
Journal of High Energy Physics | Year: 2012

Within the framework of relativistic fluctuating hydrodynamics we compute the contribution of thermal fluctuations to the effective infrared shear viscosity of a conformal fluid, focusing on quadratic (in fluctuations), second order (in velocity gradients) terms in the conservation equations. Our approach is based on the separation of hydrodynamic fields in soft and ultrasoft sectors, in which the effective shear viscosity arises due to the action of the soft modes on the evolution of the ultrasoft ones. We find that for a strongly coupled fluid with small shear viscosity-to-entropy ratio η/s the contribution of thermal fluctuations to the effective shear viscosity is small but significant. Using realistic estimates for the strongly coupled quark-gluon plasma created in heavy ion collisions, we find that for η/s close to the AdS/CFT lower bound 1/(4π) the correction is positive and at most amounts to 10% in the temperature range 200-300MeV, whereas for larger values η/s ∼ 2/(4π) the correction is negligible. For weakly coupled theories the correction is very small even for η/s = 0.08 and can be neglected. © SISSA 2012. Source

Fosco C.D.,Bariloche Atomic Center | Fosco C.D.,National University of Cuyo | Lombardo F.C.,FCEyN UBA | Mazzitelli F.D.,Bariloche Atomic Center
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

The proximity force approximation (PFA) has been widely used as a tool to evaluate the Casimir force between smooth objects at small distances. In spite of being intuitively easy to grasp, it is generally believed to be an uncontrolled approximation. Indeed, its validity has only been tested in particular examples, by confronting its predictions with the next-to-leading-order (NTLO) correction extracted from numerical or analytical solutions obtained without using the PFA. In this article we show that the PFA and its NTLO correction may be derived within a single framework, as the first two terms in a derivative expansion. To that effect, we consider the Casimir energy for a vacuum scalar field with Dirichlet conditions on a smooth curved surface described by a function ψ in front of a plane. By regarding the Casimir energy as a functional of ψ, we show that the PFA is the leading term in a derivative expansion of this functional. We also obtain the general form of the corresponding NTLO correction, which involves two derivatives of ψ. We show, by evaluating this correction term for particular geometries, that it properly reproduces the known corrections to PFA obtained from exact evaluations of the energy. © 2011 American Physical Society. Source

Manfredi J.J.,University of Pittsburgh | Parviainen M.,Aalto University | Rossi J.D.,FCEyN UBA
ESAIM - Control, Optimisation and Calculus of Variations | Year: 2012

We consider a two-player zero-sum-game in a bounded open domain Ω described as follows: at a point x Ω, Players I and II play an ε-step tug-of-war game with probability α, and with probability β (α + β = 1), a random point in the ball of radius ε centered at x is chosen. Once the game position reaches the boundary, Player II pays Player I the amount given by a fixed payoff function F. We give a detailed proof of the fact that the value functions of this game satisfy the Dynamic Programming Principle\begin{equation*} u(x) = α{2} in ol Bε(x) u (y) + y in ol Bε(x) u (y) + β kint Bε(x) u(y) ud y, end equation* for x Ω with u(y) = F(y) when y Ω. This principle implies the existence of quasioptimal Markovian strategies. © 2010 EDP Sciences, SMAI. Source

Lopez Nacir D.L.,Abdus Salam International Center For Theoretical Physics | Mazzitelli F.D.,Bariloche Atomic Center | Trombetta L.G.,FCEyN UBA
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We consider a quantum scalar field with a λφ4 interaction in curved spacetimes. The quantum effects are taken into account nonperturbatively using the Hartree approximation to the two-particle irreducible effective action. Although this approximation has been considered in many previous works, we reconsider it using a consistent nonperturbative renormalization procedure, which we extend to general curved spacetimes. We obtain the renormalized equations for the mean field and for the propagator of the fluctuations, showing explicitly their independence on the arbitrary scale introduced by the regularization scheme. We apply our results to the particular case of de Sitter spacetime and discuss spontaneous symmetry breaking. The results depend strongly on the renormalization procedure. © 2014 American Physical Society. Source

Fosco C.D.,Bariloche Atomic Center | Fosco C.D.,National University of Cuyo | Lombardo F.C.,FCEyN UBA | Mazzitelli F.D.,Bariloche Atomic Center
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We extend our previous work on a derivative expansion for the Casimir energy, to the case of the electromagnetic field coupled to two thin, imperfect mirrors. The latter are described by means of vacuum polarization tensors localized on the mirrors. We apply the results so obtained to compute the first correction to the proximity force approximation to the static Casimir effect. © 2012 American Physical Society. Source

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