Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cross sections for hard exclusive electroproduction of pi+ mesons on a
  hydrogen target   The exclusive electroproduction of pi+ mesons was studied with the Hermes
spectrometer at the DESY laboratory by scattering 27.6 GeV positron and
electron beams off an internal hydrogen gas target. The virtual-photon cross
sections were measured as a function of the Mandelstam variable t and the
squared four momentum -Q^2 of the exchanged virtual photon. A model calculation
based on Generalized Parton Distributions is in fair agreement with the data at
low values of |t| if power corrections are included. A model calculation based
on the Regge formalism gives a good description of the magnitude and the t and
Q^2 dependences of the cross section.

27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
18
The categories of this paper are: econ
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th