Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence for two-photon exchange contributions in electron-proton and
  positron-proton elastic scattering   The comparison of positron-proton and electron-proton elastic scattering
cross sections is a sensitive test for the presence of two-photon exchange
contributions. Thirty years ago, positron data were considered adequate to set
tight limits on the size of two-photon corrections. More recently, these
radiative corrections have again become a matter of great interest as a
possible explanation for the discrepancy between Rosenbluth and polarization
transfer measurements of the proton electromagnetic form factors. We have
reexamined the electron and positron scattering data to see if they can
accommodate two-photon effects of the size necessary to account for the
Rosenbluth-polarization transfer discrepancy. The data are consistent with
simple estimates of the two-photon contributions necessary to explain the
discrepancy. In fact, they strongly favor a large epsilon-dependent correction
to the positron to electron ratio, providing the first direct experimental
evidence for a two-photon contribution to unpolarized lepton-proton scattering.

18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn