Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Characteristics of geomagnetic cascading of ultra-high energy photons at
  the southern and northern sites of the Pierre Auger Observatory   Cosmic-ray photons above 10^19 eV can convert in the geomagnetic field and
initiate a preshower, i.e. a particle cascade before entering the atmosphere.
We compare the preshower characteristics at the southern and northern sites of
the Pierre Auger Observatory. In addition to a shift of the preshower patterns
on the sky due to the different pointing of the local magnetic field vectors,
the fact that the northern Auger site is closer to the geomagnetic pole results
in a different energy dependence of the preshower effect: photon conversion can
start at smaller energies, but large conversion probabilitites (>90%) are
reached for the whole sky at higher energies compared to the southern Auger
site. We show how the complementary preshower features at the two sites can be
used to search for ultra-high energy photons among cosmic rays. In particular,
the different preshower characteristics at the northern Auger site may provide
an elegant and unambiguous confirmation if a photon signal is detected at the
southern site.

13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
29
The categories of this paper are: funct-an
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
27
The categories of this paper are: comp-gas
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio