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arXiv:hep-ph/9510315

Quantum Transport Theory

Peter A. Henning

Published Oct 17, 1995• 0 citations
arXivHTMLPDF

Abstract

The problem of thermalization of a quark -- gluon plasma is addressed in the framework of thermal field theory. Within a simple approximation, the full quantum relaxation problem is solved and compared to the Boltzmann solution. Memory effects and a slowdown of the relaxation process are the results, they can be partially described by using a generalized kinetic equation without quasi-particle approximation.

DOI
10.48550/arxiv.hep-ph/9510315

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•Jan 1, 2006•2 citations
The problem of thermalization of a quark -- gluon plasma is addressed in the framework of thermal field theory. Within a simple approximation, the full quantum relaxation problem is solved and compared to the Boltzmann solution. Memory effects and a slowdown of the relaxation process are the results, they can be partially described by using a generalized kinetic equation without quasi-particle approximation.
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We study the dynamics of relaxation and thermalization in an exactly solvable model of a quantum particle interacting with a thermal bath of oscillators with the goal of understanding the effects of off-shell processes. The focus is to compare the exact evolution of the distribution function with different approximations to the relaxational dynamics: Boltzmann, non-Markovian and Markovian quantum kinetics. The time evolution of the distribution function is evaluated exactly using different approaches where each method provides different insights. There are two different cases that are studied in detail: (i) no stable particle states below threshold of the bath and a quasiparticle resonance above it and (ii) a stable discrete exact ``particle'' state below threshold. The exact solution for the evolution allows us to investigate the concept of the formation time of a quasiparticle and to study the difference between the relaxation of the distribution of particles and quasiparticles. For the case of quasiparticles in the continuum (resonances) the exact quasiparticle distribution asymptotically tends to a statistical equilibrium distribution that differs from a simple Bose-Einstein form as a result of off-shell processes such as the strength of the quasiparticle poles, the width of the unstable particle and proximity to thresholds. In case (ii), the distribution of particles does not thermalize with the bath. We study the kinetics of thermalization and relaxation by deriving a non-Markovian quantum kinetic equation which resums the perturbative series and includes off-shell effects. A Markovian approximation that includes off-shell contributions and the usual Boltzmann equation are obtained from the quantum kinetic equation in the limit of a wide separation of time scales upon different coarse-graining assumptions. The relaxational dynamics predicted by the non-Markovian, Markovian and Boltzmann approximations are compared to the exact result of the model. The Boltzmann approach is seen to fail in the case of wide resonances and when threshold and renormalization effects are important. Implications for thermalization in field theory models are discussed.
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Using the general framework of nonequilibrium statistical mechanics for relativistic quantum field systems we derive the basic equations of quantum field kinetics. The main aim of the approach is calculation of observables associated with quark-gluon plasma which is out of thermal and chemical equilibrium. We show that in the regime of high rate of the phase and statistical mixing the perturbation theory for many-body quantum field system should have the form which much differs from the standard expansion in powers of coupling constant. ( Figures can be obtained from the author )
Boltzmann equations are often used to study the thermal evolution of particle reaction networks. Prominent examples are the computation of the baryon asymmetry of the universe and the evolution of the quark-gluon plasma after a relativistic heavy ion collision. However, Boltzmann equations are only a classical approximation of the quantum thermalization process which is described by the so-called Kadanoff-Baym equations. This raises the question how reliable Boltzmann equations are as approximations to the full Kadanoff-Baym equations. Therefore, we present in this thesis a detailed comparison of Boltzmann and Kadanoff-Baym equations in the framework of relativistic quantum field theories in 3+1 space-time dimensions. In a first step, for simplicity we consider a real scalar Phi^4 quantum field theory and in a second step we generalize our results to a chirally invariant Yukawa-type quantum field theory including fermions. The obtained numerical solutions reveal significant discrepancies in the results predicted by both types of equations. Apart from quantitative discrepancies, on a qualitative level the universality respected by Kadanoff-Baym equations is severely restricted in the case of Boltzmann equations. Furthermore, Kadanoff-Baym equations strongly separate the time scales between kinetic and chemical equilibration. This separation of time scales is absent for Boltzmann equations.