arXiv subjects

E. S. Fraga

Publications and source records attributed to E. S. Fraga.

At least 37 records · Page 2Linked to original sources

arXiv2009

Non-Markovian expansion in quantum dissipative systems

We consider the non-Markovian Langevin evolution of a dissipative dynamical system in quantum mechanics in the path integral formalism. After discussing the role of the frequency cutoff for the interaction of the system with the heat bath and the kernel and noise correlator that follow from the most common choices, we derive an analytic expansion for the exact non-Markovian dissipation kernel and the corresponding colored noise in the general case that is consistent with the fluctuation-dissipation theorem and incorporates systematically non-local corrections. We illustrate the modifications to results obtained using the traditional (Markovian) Langevin approach in the case of the exponential kernel and analyze the case of the non-Markovian Brownian motion.

quant-ph↗

arXiv2009

Phase conversion in a weakly first-order quark-hadron transition

We investigate the process of phase conversion in a thermally-driven {\it weakly} first-order quark-hadron transition. This scenario is physically appealing even if the nature of this transition in equilibrium proves to be a smooth crossover for vanishing baryonic chemical potential. We construct an effective potential by combining the equation of state obtained within Lattice QCD for the partonic sector with that of a gas of resonances in the hadronic phase, and present numerical results on bubble profiles, nucleation rates and time evolution, including the effects from reheating on the dynamics for different expansion scenarios. Our findings confirm the standard picture of a cosmological first-order transition, in which the process of phase conversion is entirely dominated by nucleation, also in the case of a weakly first-order transition. On the other hand, we show that, even for expansion rates much lower than those expected in high-energy heavy ion collisions, nucleation is very unlikely, indicating that the main mechanism of phase conversion is spinodal decomposition. Our results are compared to those obtained for a strongly first-order transition, as the one provided by the MIT bag model.

hep-ph↗

arXiv2009

Quark mass and isospin dependence of the deconfining critical temperature

We describe the deconfining critical temperature dependence on the pion mass and on the isospin chemical potential in remarkably good agreement with lattice data. Our framework incorporates explicit dependence on quark masses, isospin and baryonic chemical potentials for the case of two flavors through ingredients from well-known high- and low-energy theories. In the low-energy sector, the system is described by a minimal chiral perturbation theory effective action, corresponding to a hot gas of pion quasiparticles and heavy nucleons. For the high-temperature sector we adopt a simple extension of the fuzzy bag model. We also briefly discuss the effects of mass asymmetry and baryon chemical potential.

hep-ph↗

arXiv2008

Zero-mode analysis of quantum statistical physics

We present a unified formulation for quantum statistical physics based on the representation of the density matrix as a functional integral. We identify the stochastic variable of the effective statistical theory that we derive as a boundary configuration and a zero mode relevant to the discussion of infrared physics. We illustrate our formulation by computing the partition function of an interacting one-dimensional quantum mechanical system at finite temperature from the path-integral representation for the density matrix. The method of calculation provides an alternative to the usual sum over periodic trajectories: it sums over paths with coincident endpoints, and includes non-vanishing boundary terms. An appropriately modified expansion into Matsubara modes provides a natural separation of the zero-mode physics. This feature may be useful in the treatment of infrared divergences that plague the perturbative approach in thermal field theory.

quant-ph↗

arXiv2008

Semiclassical calculation of decay rates

Several relevant aspects of quantum-field processes can be well described by semiclassical methods. In particular, the knowledge of non-trivial classical solutions of the field equations, and the thermal and quantum fluctuations around them, provide non-perturbative information about the theory. In this work, we discuss the calculation of the one-loop effective action from the semiclasssical viewpoint. We intend to use this formalism to obtain an accurate expression for the decay rate of non-static metastable states.

hep-ph↗

arXiv2008

Mass and chemical asymmetry in QCD matter

We consider two-flavor asymmetric QCD combined with a low-energy effective model inspired by chiral perturbation theory and lattice data to investigate the effects of masses, isospin and baryon number on the pressure and the deconfinement phase transition. Remarkable agreement with lattice results is found for the critical temperature behavior. Further analyses of the cold, dense case and the influence of quark mass asymmetry are also presented.

hep-ph↗

arXiv2008

On the nucleation of hadronic domains in the quark-hadron transition

We present numerical results on bubble profiles, nucleation rates and time evolution for a weakly first-order quark-hadron phase transition in different expansion scenarios. We confirm the standard picture of a cosmological first-order phase transition, in which the phase transition is entirely dominated by nucleation. We also show that, even for expansion rates much lower than those expected in heavy-ion collisions nucleation is very unlikely, indicating that the main phase conversion mechanism is spinodal decomposition.

hep-ph↗

arXiv2007

Langevin dynamics of the pure SU(2) deconfining transition

We investigate the dissipative real-time evolution of the order parameter for the deconfining transition in the pure SU(2) gauge theory. The approach to equilibrium after a quench to temperatures well above the critical one is described by a Langevin equation. To fix completely the markovian Langevin dynamics we choose the dissipation coefficient, that is a function of the temperature, guided by preliminary Monte Carlo simulations for various temperatures. Assuming a relationship between Monte Carlo time and real time, we estimate the delay in thermalization brought about by dissipation and noise.

hep-ph↗

arXiv2007

Semiclassical thermodynamics of scalar fields

We present a systematic semiclassical procedure to compute the partition function for scalar field theories at finite temperature. The central objects in our scheme are the solutions of the classical equations of motion in imaginary time, with spatially independent boundary conditions. Field fluctuations -- both field deviations around these classical solutions, and fluctuations of the boundary value of the fields -- are resummed in a Gaussian approximation. In our final expression for the partition function, this resummation is reduced to solving certain ordinary differential equations. Moreover, we show that it is renormalizable with the usual 1-loop counterterms.

hep-ph↗

arXiv2006

QCD Effective action at high temperature and small chemical potential

We present a construction of an effective Yang-Mills action for QCD, from the expansion of the fermionic determinant in terms of powers of the chemical potential at high temperature, for the case of massless quarks. We analyze this expansion in the perturbative region and find that it gives extra spurious information. We propose for the non-perturbative sector a simplified effective action which, in principle, contains only the relevant information.

hep-ph↗

arXiv2006

Dissipation and memory effects in pure glue deconfinement

We investigate the effects of dissipation in the deconfining transition for a pure SU(2) gauge theory. Using an effective model for the order parameter, we study its Langevin evolution numerically, and compare results from local additive noise dynamics to those obtained considering an exponential non-local kernel for early times.

hep-ph↗

arXiv2005

Hydrodynamical instabilities in an expanding quark gluon plasma

We study the mechanism responsible for the onset of instabilities in a chiral phase transition at nonzero temperature and baryon chemical potential. As a low-energy effective model, we consider an expanding relativistic plasma of quarks coupled to a chiral field, and obtain a phenomenological chiral hydrodynamics from a variational principle. Studying the dispersion relation for small fluctuations around equilibrium, we identify the role played by chiral waves and pressure waves in the generation of instabilities. We show that pressure modes become unstable earlier than chiral modes.

nucl-th↗

arXiv2005

Quasiparticle model for deconfined matter and the nucleation of hadrons

Assuming a first-order chiral transition scenario, we study the process of phase conversion driven by homogeneous nucleation. We adopt a quasiparticle model whose parameters are fit to lattice QCD data to describe the pressure at high temperature in the deconfined sector, and a bag model equation of state for pions in the low-temperature sector. We compute the critical radius and nucleation rate in the thin-wall approximation, and compare the results to the ones obtained using the bag model and the linear $σ$ model.

hep-ph↗

arXiv2004

Correlations around an interface

We compute one-loop correlation functions for the fluctuations of an interface using a field theory model. We obtain them from Feynman diagrams drawn with a propagator which is the inverse of the Hamiltonian of a Poschl-Teller problem. We derive an expression for the propagator in terms of elementary functions, show that it corresponds to the usual spectral sum, and use it to calculate quantities such as the surface tension and interface profile in two and three spatial dimensions. The three-dimensional quantities are rederived in a simple, unified manner, whereas those in two dimensions extend the existing literature, and are applicable to thin films. In addition, we compute the one-loop self-energy, which may be extracted from experiment, or from Monte Carlo simulations. Our results may be applied in various scenarios, which include fluctuations around topological defects in cosmology, supersymmetric domain walls, Z(N) bubbles in QCD, domain walls in magnetic systems, interfaces separating Bose-Einstein condensates, and interfaces in binary liquid mixtures.

hep-ph↗

arXiv2003

The Equation of State for Cold and Dense Strongly Interacting Matter

We discuss recent results for the equation of state for cold and dense strongly interacting matter. We consider the extreme cases of very high densities, where weak-coupling approaches may in principle give reasonable results, and very low densities, where we use the framework of heavy-baryon chiral perturbation theory. We also speculate on the nature of the chiral transition and present possible astrophysical implications.

nucl-th↗

arXiv2003

Dilute Nuclear Matter in Chiral Perturbation Theory

We use chiral perturbation theory to compute the effective nucleon propagator in an expansion about low density in the chiral limit. We neglect four-nucleon interactions and focus on pion exchange. Evaluating the nucleon self-energy on its mass shell to leading order, we show that the effective nucleon mass increases by a small amount. We discuss the relevance of our results to the structure of compact stars.

nucl-th↗

arXiv2002

Improved semiclassical density matrix: taming caustics

We present a simple method to deal with caustics in the semiclassical approximation to the thermal density matrix of a particle moving on the line. For simplicity, only its diagonal elements are considered. The only ingredient we require is the knowledge of the extrema of the Euclidean action. The procedure makes use of complex trajectories, and is applied to the quartic double-well potential.

quant-ph↗

arXiv2001

Microscopic Evolution of a Weakly Interacting Homogeneous Bose Gas

We provide a detailed description of the nonequilibrium time evolution of an interacting homogeneous Bose-Einstein condensate. We use a nonperturbative in-medium quantum field theory approach as a microscopic model for the Bose gas. The real-time dynamics of the condensate is encoded in a set of self-consistent equations which corresponds to an infinite sum of ladder Feynman diagrams. The crucial role played by the interaction between fluctuations for the instability generation is thoroughly described.

cond-mat.soft↗