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arXiv:2210.14281

Kinetic coefficients in the formalism of time-dependent Green's functions at finite temperature

Viacheslav Aleksandrovich Krivorol, Mikhail Yu. Nalimov

Published Oct 25, 2022• 0 citations
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Abstract

We discuss the microscopical justification of dissipation in the model nonrelativistic Fermi and Bose systems with weak local interactions above phase transitions. The dynamics of equilibrium fluctuations are considered in Keldysh - Schwinger framework. We show that the dissipation is related to pinch singularities of the diagram technique. Using Dyson - Schwinger equation and the two-loop approximation we define and calculate the attenuation parameter which is related to exponentiality of Green's functions decay. We show that the attenuation parameter is the microscopic analogue of the Onsager kinetic coefficient and it is related to attenuation in the excitation spectrum.

DOI
10.48550/arxiv.2210.14281

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The equation of motion for nonequilibrium Green functions is derived within the framework of the Schwinger and Keldysh formalism of perturbation expansion. For nonequilibrium distribution Green functions, the equation of motion derived from quantum mechanics contains undefined singularities, whose explicit form depends on the specific initial or boundary condition. In the present work, the exact expression of singular terms is found in the equation of motion from the time-looped perturbation theory in which the adiabatic initial condition is implied. Unlike the usual Dyson perturbation formalism or the well known Kadanoff-Baym equation of motion, our resulting equation can be adopted directly for calculations without the graphical analysis, which depends on the specific form of the Hamiltonian. On the basis of this equation of motion, the procedure of a nonperturbative solution is outlined and potential applications are briefly discussed.
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The search for the conjectured QCD critical point in heavy-ion collisions requires to account for far-from equilibrium effects as well as fluctuations, and in particular non-Gaussian fluctuations, in the modeling of the dynamics of the hot and dense matter created in such collisions. In order to study far-from equilibrium effects as well as fluctuations, in this work we construct a Schwinger-Keldysh effective field theory (EFT) for the diffusion of the density to all orders in derivatives. The dissipation in the free part of our EFT follows the Boltzmann equation in the relaxation-time approximation (RTA). The interaction part of the EFT is constructed based on the self-interaction of the density field. We analytically find the quadratic and cubic parts of the KMS-invariant EFT in closed form, resummed in derivatives. We then explicitly compute the symmetrized three-point function at tree level, and investigate its analytical structure in detail. We also analytically calculate the branch-point singularity that appears in the structure of the two-point response function due to loop effects. Our results are important for future studies of the real-time dynamics of the correlation functions and the possible relation to thermalization when the system is far from equilibrium.