Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Two-photon exchange and polarization physics in electron-proton
  scattering   We discuss effects of two-photon exchange (TPE) in various observables of the
elastic ep-scattering. The imaginary part of the TPE amplitude manifests in
target and beam normal spin asymmetries. The real part contributes to the
cross-section. A model-independent phenomenological analysis of experimental
data is performed. Its results are compared with theoretical calculations.

23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
2
The categories of this paper are: math
3
The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
36
The categories of this paper are: cmp-lg
25
The categories of this paper are: patt-sol
14
The categories of this paper are: math-ph
19
The categories of this paper are: eess
7
The categories of this paper are: hep-th
1
The categories of this paper are: cs