Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Upper limit on the cosmic-ray photon fraction at EeV energies from the
  Pierre Auger Observatory   From direct observations of the longitudinal development of ultra-high energy
air showers performed with the Pierre Auger Observatory, upper limits of 3.8%,
2.4%, 3.5% and 11.7% (at 95% c.l.) are obtained on the fraction of cosmic-ray
photons above 2, 3, 5 and 10 EeV (1 EeV = 10^18 eV) respectively. These are the
first experimental limits on ultra-high energy photons at energies below 10
EeV. The results complement previous constraints on top-down models from array
data and they reduce systematic uncertainties in the interpretation of shower
data in terms of primary flux, nuclear composition and proton-air
cross-section.

24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas