Kadanoff-Baym Theory for Thermalization of Quantum Fields
Akihiro Nishiyama
We present a theoretical study of thermalization of quantum fields with Kadanoff-Baym (KB) equation. First we introduce a field theoretical technique known as 2 Particle Irreducible (2PI) effective action and review derivation of equation of motion for mean fields and fluctuations, then derive Kadanoff-Baym equation in scalar and gauge theories. In order to analyze thermalization processes we introduce relativistic entropy current based on the first order gradient expansion of Kadanoff-Baym equation. Then we show that in taking into account next-to-leading order (NLO) self-energy of the coupling expansion in φ4 theory (in symmetric phase <φ^> = 0), it is possible to prove the H-theorem within the 1st order gradient expansion. We also show that it is possible to prove the H-theorem for KB equation with NLO self-energy of 1/N expansion in O(N) theory (in symmetric phase). Furthermore we suggest a possibility for H-theorem to be satisfied in KB equation with leading-order (LO) self-energy of the coupling expansion in non-Abelian gauge theory. In d+1 dimensions (d＞2) LO particle number changing processes, such as 0-to-3, 1-to-2 and 2-to-1, which are prohibited in the normal Boltzmann approach with on-shell particles, may contribute to entropy production in KB approach although we must deal with gauge dependence and infrared divergence to complete the proof. Next we perform numerical simulations in spatially homogeneous configurations to investigate thermalization properties of the system in 1+1 and 2+1 dimensions for φ4 theory with NLO self-energy of the coupling expansion and 1+1 dimensions for O(N) theory with NLO self-energy of 1/N expansion. We also estimate the time evolution of the kinetic entropy in the case of scalar φ4 and O(N) theory. Then we find that at later times X0≫1/m, where m represents mass of particles, the kinetic entropy increases monotonically and approaches the equilibrium value, although the limited time interval at the earlier times X0～1/m invalidates the use of it. No thermalization occurs in the simulations with the normal Boltzmann equation with 2-to-2 collision term in 1+1 dimensions, while it occurs in the simulations with KB equation in 1+1 dimensions. It is due to two types of offshell effects: memory effects and spectral functions with the decay width.
•Mar 24, 2010•0 citations