Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Boltzmann Equation from Quantum Field Theory   We show from first principles the emergence of classical Boltzmann equations
from relativistic nonequilibrium quantum field theory as described by the
Kadanoff-Baym equations. Our method applies to a generic quantum field, coupled
to a collection of background fields and sources, in a homogeneous and
isotropic spacetime. The analysis is based on analytical solutions to the full
Kadanoff-Baym equations, using the WKB approximation. This is in contrast to
previous derivations of kinetic equations that rely on similar physical
assumptions, but obtain approximate equations of motion from a gradient
expansion in momentum space. We show that the system follows a generalized
Boltzmann equation whenever the WKB approximation holds. The generalized
Boltzmann equation, which includes off-shell transport, is valid far from
equilibrium and in a time dependent background, such as the expanding universe.

31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
25
The categories of this paper are: patt-sol
11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
28
The categories of this paper are: alg-geom
19
The categories of this paper are: eess
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
29
The categories of this paper are: funct-an