Nonequilibrium dynamics of quantum fields

Calculations of nonequilibrium processes become increasingly feasable in quantum field theory from first principles. There has been important progress in our analytical understanding based on 2PI generating functionals. In addition, for the first time direct lattice simulations based on stochastic quantization techniques have been achieved. The quantitative descriptions of characteristic far-from-equilibrium time scales and thermal equilibration in quantum field theory point ...

We consider the time evolution of nonequilibrium quantum scalar fields in the O(N) model, using the next-to-leading order 1/N expansion of the 2PI effective action. A comparison with exact numerical simulations in 1+1 dimensions in the classical limit shows that the 1/N expansion gives quantitatively precise results already for moderate values of N. For sufficiently high initial occupation numbers the time evolution of quantum fields is shown to be accurately described by cla...

We present a general and model-independent method to obtain an effective Markovian quantum kinetic equation for the expectation value of a slowly evolving scalar field in an adiabatically evolving background from first principles of nonequilibrium quantum field theory. The method requires almost no assumptions about the field's interactions and the composition of the background, except that 1) the coupling constants shall be small enough for perturbation theory to be applicab...

Nonperturbative dynamics of quantum fields out of equilibrium is often described by the time evolution of a hierarchy of correlation functions, using approximation methods such as Hartree, large N, and nPI-effective action techniques. These truncation schemes can be implemented equally well in a classical statistical system, where results can be tested by comparison with the complete nonlinear evolution obtained by numerical methods. For a 1+1 dimensional scalar field we find...

There has been substantial progress in recent years in the quantitative understanding of the nonequilibrium time evolution of quantum fields. Important topical applications, in particular in high energy particle physics and cosmology, involve dynamics of quantum fields far away from the ground state or thermal equilibrium. In these cases, standard approaches based on small deviations from equilibrium, or on a sufficient homogeneity in time underlying kinetic descriptions, are...

Lecture notes of the Les Houches Summer School on 'Strongly interacting quantum systems out of equilibrium'.

The quantum dynamics of the symmetry broken \lambda (\Phi^2)^2 scalar field theory in the presence of an homogeneous external field is investigated in the large N limit. We consider an initial thermal state of temperature T for a constant external field J. A subsequent sign flip of the external field, J to -J, gives rise to an out of equilibrium nonperturbative quantum field dynamics. We review here the dynamics for the symmetry broken lambda(\Phi^2)^2 scalar N component fiel...

Relaxation process of a coherent scalar field oscillation in the thermal bath is investigated using nonequilibrium quantum field theory. The Langevin-type equation of motion is obtained which has a memory term and both additive and multiplicative noise terms. The dissipation rate of the oscillating scalar field is calculated for various interactions such as Yukawa coupling, three-body scalar interaction, and biquadratic interaction. When the background temperature is larger t...

We study the detailed out of equilibrium time evolution of a homogeneous Bose-Einstein condensate.We consider a nonrelativistic quantum theory for a self-interacting complex scalar field, immersed in a thermal bath, as an effective microscopic model for the description of the Bose-Einstein condensate. This approach yields the following main results:(i) the interaction between fluctuations proves to be crucial in the mechanism of instability generation; (ii) there are essentia...

I review the use of the 2PI effective action in nonequilibrium quantum field theory. The approach enables one to find approximation schemes which circumvent long-standing problems of non-thermal or secular (unbounded) late-time evolutions encountered in standard loop or 1/N expansions of the 1PI effective action. It is shown that late-time thermalization can be described from a numerical solution of the three-loop 2PI effective action for a scalar $\phi^4$--theory in 1+1 dime...