Navegando por Autor "Fraga, Eduardo Souza"
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Artigo Effective potential in the boundary effective theory formalism(American Physical Society, 2011-06-10) Moreira, André Bessa; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo Souza; Gelis, FrancoisWe calculate the one-loop effective potential at finite temperature for a system of massless scalar fields with quartic interaction λ ϕ 4 in the framework of the boundary effective theory formalism. The calculation relies on the solution of the classical equation of motion for the field and Gaussian fluctuations around it. Our result is nonperturbative and differs from the standard one-loop effective potential for field values larger than T/ √λArtigo Functional density matrix formulation of quantum statistics(American Physical Society, 2010-01-05) Moreira, André Bessa; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo SouzaWe present a unified formulation for quantum statistical physics based on the representation of the density matrix as a functional integral. For quantum statistical (thermal) field theory, the stochastic variable of the statistical theory is a boundary field configuration. We explore the properties of an effective theory for such boundary configurations and apply it to the computation of the partition function of an interacting one-dimensional quantum-mechanical system at finite temperature. Plots of free energy and specific heat show excellent agreement with more involved semiclassical results. The method of calculation provides an alternative to the usual sum over periodic trajectories: it sums over paths with coincident end points and includes nonvanishing boundary terms. An appropriately modified expansion into modified Matsubara modes is presentedArtigo On the nucleation of hadronic domains in the quark-hadron transition(Elsevier, 2009-04-01) Mintz, Bruno Werneck; Moreira, André Bessa; Fraga, Eduardo SouzaWe 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 decompositionArtigo Phase conversion in a weakly first-order quark-hadron transition(American Physical Society, 2009-02-12) Moreira, André Bessa; Fraga, Eduardo Souza; Mintz, Bruno WerneckWe investigate the process of phase conversion in a thermally driven 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 modelArtigo Pressure of massless hot scalar theory in the boundary effective theory framework(American Physical Society, 2011-04-22) Moreira, André Bessa; Brandt, Fernando Tadeu Caldeira; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo SouzaWe use the boundary effective theory approach to thermal field theory in order to calculate the pressure of a system of massless scalar fields with quartic interaction. The method naturally separates the infrared physics, and is essentially nonperturbative. To lowest order, the main ingredient is the solution of the free Euler-Lagrange equation with nontrivial (time) boundary conditions. We derive a resummed pressure, which is in good agreement with recent calculations found in the literature, following a very direct and compact procedureArtigo Quantum statistical correlations in thermal field theories: boundary effective theory(American Physical Society, 2010-09-08) Moreira, André Bessa; Brandt, Fernando Tadeu Caldeira; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo SouzaWe show that the one-loop effective action at finite temperature for a scalar field with quartic interaction has the same renormalized expression as at zero temperature if written in terms of a certain classical field ϕc, and if we trade free propagators at zero temperature for their finite-temperature counterparts. The result follows if we write the partition function as an integral over field eigenstates (boundary fields) of the density matrix element in the functional Schrödinger field representation, and perform a semiclassical expansion in two steps: first, we integrate around the saddle point for fixed boundary fields, which is the classical field ϕc, a functional of the boundary fields; then, we perform a saddle-point integration over the boundary fields, whose correlations characterize the thermal properties of the system. This procedure provides a dimensionally reduced effective theory for the thermal system. We calculate the two-point correlation as an exampleArtigo Semiclassical calculation of decay rates(Elsevier, 2009-04-01) Moreira, André Bessa; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo SouzaSeveral 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 statesArtigo Semiclassical partition function for the double-well potential(American Physical Society, 2014-07-15) Fogaça, Daniel Kroff; Moreira, André Bessa; Carvalho Filho, Carlos Alberto Aragão de; Fraga, Eduardo Souza; Jorás, Sergio Eduardo de Carvalho EyerWe compute the partition function and specific heat for a quantum-mechanical particle under the influence of a quartic double-well potential nonperturbatively, using the semiclassical method. Near the region of bounded motion in the inverted potential, the usual quadratic approximation fails due to the existence of multiple classical solutions and caustics. Using the tools of catastrophe theory, we identify the relevant classical solutions, showing that at most two have to be considered. This corresponds to the first step towards the study of spontaneous symmetry breaking and thermal phase transitions in the nonperturbative framework of the boundary effective theory