Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Semiclassical Approximation meets Keldysh-Schwinger diagrammatic
  technique: Scalar $\varphi^4$   We study the evolution of the non-equilibrium quantum fields from a highly
excited initial state in two approaches: the standard Keldysh-Schwinger diagram
technique and the semiclassical expansion. We demonstrate explicitly that these
two approaches coincide if the coupling constant $g$ and the Plank constant
$\hbar$ are small simultaneously. Also, we discuss loop diagrams of the
perturbative approach, which are summed up by the leading order term of the
semiclassical expansion. As an example, we consider shear viscosity for the
scalar field theory at the leading semiclassical order. We introduce the new
technique that unifies both semiclassical and diagrammatic approaches and open
the possibility to perform the resummation of the semiclassical contributions.

27
The categories of this paper are: comp-gas
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
21
The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
24
The categories of this paper are: dg-ga
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
19
The categories of this paper are: eess