Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An upper limit to photons from first data taken by the Pierre Auger
  Observatory   Many models for ultra-high energy cosmic rays postulate exotic scenarios to
explain the sources or the nature of these particles. A characteristic feature
of these models is the prediction of a significant flux of photons at
ultra-high energy. The Pierre Auger Observatory offers a great potential to
search for such photons. We present shower observables with sensitivity to
photons and the search strategy employed. An upper limit to photon primaries is
derived from first Auger data. Prospects for constraining theoretical source
models are discussed.

36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
29
The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs