Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Pion re-scattering operator in the S-matrix approach   The pion re-scattering operator for pion production, derived recently in
time-ordered perturbation theory, is compared with the one following from the
simple S-matrix construction. We show that this construction is equivalent to
the on-shell approximation introduced in previous papers. For a realistic NN
interaction, the S-matrix approach, and its simplified fixed
threshold-kinematics version, work well near threshold.

16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
34
The categories of this paper are: chem-ph
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
37
The categories of this paper are: supr-con
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex