Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Preserving the gauge invariance of meson production currents in the
  presence of explicit final-state interactions   A comprehensive formalism is developed to preserve the gauge invariance of
currents describing the photo- or electroproduction of mesons off the nucleon
when the final-state interactions of mesons and nucleons is taken into account
explicitly. Replacing exchange currents by auxiliary currents, it is found that
all contributions due to explicit final-state interactions are purely
transverse and do not contain a Kroll-Ruderman-type contact current. The
relation of the present formulation to tree-level-type prescriptions is shown.

33
The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga
14
The categories of this paper are: math-ph
3
The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
30
The categories of this paper are: q-alg