Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Measurement of two-photon exchange effect by comparing elastic $e^\pm p$
  cross sections   [Background] The electromagnetic form factors of the proton measured by
unpolarized and polarized electron scattering experiments show a significant
disagreement that grows with the squared four momentum transfer ($Q^{2}$).
Calculations have shown that the two measurements can be largely reconciled by
accounting for the contributions of two-photon exchange (TPE). TPE effects are
not typically included in the standard set of radiative corrections since
theoretical calculations of the TPE effects are highly model dependent, and,
until recently, no direct evidence of significant TPE effects has been
observed.
  [Purpose] We measured the ratio of positron-proton to electron-proton
elastic-scattering cross sections in order to determine the TPE contribution to
elastic electron-proton scattering and thereby resolve the proton electric form
factor discrepancy.
  [Methods] We produced a mixed simultaneous electron-positron beam in
Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a
radiator to produce a bremsstrahlung photon beam and then passing the photon
beam through a convertor to produce electron/positron pairs. The mixed
electron-positron (lepton) beam with useful energies from approximately 0.85 to
3.5 GeV then struck a 30-cm long liquid hydrogen (LH$_2$) target located within
the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered
leptons and the recoiling protons we identified and reconstructed elastic
scattering events and determined the incident lepton energy. A detailed
description of the experiment is presented.

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The categories of this paper are: cond-mat
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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: supr-con
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: bayes-an
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The categories of this paper are: chao-dyn
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The categories of this paper are: patt-sol
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The categories of this paper are: comp-gas
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The categories of this paper are: q-bio
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The categories of this paper are: mtrl-th
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The categories of this paper are: econ
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The categories of this paper are: dg-ga
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The categories of this paper are: alg-geom
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The categories of this paper are: acc-phys