Buenos Aires, Argentina
Buenos Aires, Argentina

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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.

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.

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.

Villar P.I.,FCEyN UBA | Lombardo F.C.,FCEyN UBA
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We compute the geometric phase for a spin-1/2 particle under the presence of a composite environment, composed of an external bath (modeled by an infinite set of harmonic oscillators) and another spin-1/2 particle. We consider both cases: an initial entanglement between the spin-1/2 particles and an initial product state in order to see if the initial entanglement has an enhancement effect on the geometric phase of one of the spins. We follow the nonunitary evolution of the reduced density matrix and evaluate the geometric phase for a single two-level system. We also show that the initial entanglement enhances the sturdiness of the geometric phase under the presence of an external composite environment. © 2011 American Physical Society.

Lombardo F.C.,FCEyN UBA | Villar P.I.,FCEyN UBA
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We describe the decoherence process induced on a two-level quantum system in direct interaction with a nonequilibrium environment. The nonequilibrium feature is represented by a nonstationary random function corresponding to the fluctuating transition frequency between two quantum states coupled to the surroundings. In this framework, we compute the decoherence factors which have a characteristic "dip" related to the initial phases of the bath modes. We therefore study different types of environments, namely, ohmic and supraohmic. These environments present different decoherence time scales than the thermal environment we used to study. As a consequence, we compute analytically and numerically the nonunitary geometric phase for the qubit in a quasicyclic evolution under the presence of these particular nonequilibrium environments. We show in which cases decoherence effects could, in principle, be controlled in order to perform a measurement of the geometric phase using standard procedures. © 2013 American Physical Society.

Lombardo F.C.,FCEyN UBA | Turiaci G.J.,FCEyN UBA
Physical Review Letters | Year: 2012

We study the process of decoherence in acoustic black holes. We focus on the ion trap model proposed by Horstmann et al., but the formalism is general to any experimental implementation. For that particular setup, we compute the decoherence time for the experimental parameters that they proposed. We find that a quantum to classical transition occurs during the measurement, and we propose improved parameters to avoid such a feature. We also study the entanglement between the Hawking-pair phonons for an acoustic black hole while in contact with a reservoir, through the quantum correlations, showing that they remain strongly correlated for small enough times and temperatures. © 2012 American Physical Society.

Bianchi M.S.,Humboldt University of Berlin | Giribet G.,FCEyN UBA | Leoni M.,FCEyN UBA | Penati S.,University of Milan Bicocca
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We compute the expectation value of the 1/2 BPS circular Wilson loop in N=6 superconformal Chern-Simons (ABJM) theory at two loops in perturbation theory. The result shows perfect agreement with the prediction from localization and the proposed framing factor. © 2013 American Physical Society.

Comas G.,FCEyN UBA | Seiguer M.,FCEyN UBA
Foundations of Computational Mathematics | Year: 2011

We describe in the space of binary forms of degree d the strata of forms having a given rank. We also give a simple algorithm for determining the rank of a given form. © 2010 SFoCM.

Peralta-Ramos J.,São 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.

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.

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