Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: General analysis of two--photon exchange in elastic electron--$^4He$
  scattering and $e^++e^-\to \pi^++\pi^-.$   Using a general parametrization of the spin structure of the matrix element
for the elastic $e^-+^4He$ scattering and for the annihilation $e^++e^-\to
\pi^++\pi^-$ reactions in terms of two complex amplitudes, we derive general
properties of the observables in presence of two--photon exchange. We show that
this mechanism induces a specific dependence of the differential cross section
on the angle of the emitted particle. We reanalyze the existing experimental
data on the differential cross section, for elastic electron scattering on
$^4He$, in the light of this result.

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The categories of this paper are: physics
10
The categories of this paper are: q-bio
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
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The categories of this paper are: astro-ph
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The categories of this paper are: gr-qc
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
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The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
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The categories of this paper are: econ
24
The categories of this paper are: dg-ga