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Register a new userReal-Time Kadanoff-Baym Approach to Nuclear Response Functions
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Linear response functions are calculated for symmetric nuclear matter of normal density by time-evolving two-time Greens functions with conserving self-energy insertions, thereby satisfying the energy-sum rule. Nucleons are regarded as moving in a mean field defined by an effective mass. A two-body effective (or residual) interaction, represented by a gaussian local interaction, is used to find the effect of correlations in a second order as well as a ring approximation. The response function S(e,q) is calculated for 0.2<q<1.2 fm^{-1}. Comparison is made with the nucleons being un-correlated, RPA+HF only.
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Linear density response functions are calculated for symmetric nuclear matter of normal density by time-evolving two-time Greens functions in real time. Of particular interest is the effect of correlations. The system is therefore initially time-evolved with a collision term calculated in a direct Born approximation as well as with full (RPA) ring-summation until fully correlated. An external time-dependent potential is then applied. The ensuing density fluctuations are recorded to calculate the density response. This method was previously used by Kwong and Bonitz for studying plasma oscillations in a correlated electron gas. The energy-weighted sum-rule for the response function is guaranteed by using conserving self-energy insertions as the method then generates the full vertex-functions. These can alternatively be calculated by solving a Bethe -Salpeter equation as done in some previous works. The (first order) mean field is derived from a momentum-dependent (non-local) interaction while $2^{nd}$ order self-energies are calculated using a particle-hole two-body effective (or residual) interaction given by a gaussian it local rm potential. We present numerical results for the response function $S(omega,q_0)$ for $q_0=0.2,0.4$ and $0.8 {rm fm}^{-1}$. Comparison is made with the nucleons being un-correlated i.e. with only the first order mean field included, the HF+RPA approximation. We briefly discuss the relation of our work with the Landau quasi-particle theory as applied to nuclear systems by Sjoberg and followers using methods developped by Babu and Brown, with special emphasis on the induced interaction.
This review provides a written version of the lectures presented at the Schladming Winter School 2008, Austria, on Nonequilibrium Aspects of Quantum Field Theory. In particular, it shows the way from quantum-field theory - in two-particle irreducible approximation - to the Kadanoff-Baym (KB) equations and various approximations schemes of the KB equations in phase space. This ultimately leads to the formulation of an off-shell transport theory that well incorporates the underlying quantum physics. Remarkably, these transport equations may be solved within a testparticle representation that allows to study non-equilibrium quantum systems in the weak and strong coupling regime. Actual applications to dilepton production in heavy-ion reactions are presented in comparison with available data. The approach, furthermore, allows to address the hadronization process from partonic to hadronic degrees of freedom.
Nonequilibrium Greens functions represent underutilized means of studying the time evolution of quantum many-body systems. In view of a rising computer power, an effort is underway to apply the Greens functions formalism to the dynamics of central nuclear reactions. As the first step, mean-field evolution for the density matrix for colliding slabs is studied in one dimension. The strategy to extend the dynamics to correlations is described.
We derive Boltzmann equations for massive spin-1/2 fermions with local and nonlocal collision terms from the Kadanoff--Baym equation in the Schwinger--Keldysh formalism, properly accounting for the spin degrees of freedom. The Boltzmann equations are expressed in terms of matrix-valued spin distribution functions, which are the building blocks for the quasi-classical parts of the Wigner functions. Nonlocal collision terms appear at next-to-leading order in $hbar$ and are sources for the polarization part of the matrix-valued spin distribution functions. The Boltzmann equations for the matrix-valued spin distribution functions pave the way for simulating spin-transport processes involving spin-vorticity couplings from first principles.
Basic issues of the time-dependent density-functional theory are discussed, especially on the real-time calculation of the linear response functions. Some remarks on the derivation of the time-dependent Kohn-Sham equations and on the numerical methods are given.
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