Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultra High-Energy Cosmic Ray Observations   The year 2007 has furnished us with outstanding results about the origin of
the most energetic cosmic rays: a flux suppression as expected from the
GZK-effect has been observed in the data of the HiRes and Auger experiments and
correlations between the positions of nearby AGN and the arrival directions of
trans-GZK events have been observed by the Pierre Auger Observatory. The latter
finding marks the beginning of ultra high-energy cosmic ray astronomy and is
considered a major breakthrough starting to shed first light onto the sources
of the most extreme particles in nature. This report summarizes those
observations and includes other major advances of the field, mostly presented
at the 30th International Cosmic Ray Conference held in Merida, Mexico, in July
2007. With increasing statistics becoming available from current and even
terminated experiments, systematic differences amongst different experiments
and techniques can be studied in detail which is hoped to improve our
understanding of experimental techniques and their limitations.

20
The categories of this paper are: acc-phys
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
27
The categories of this paper are: comp-gas
21
The categories of this paper are: adap-org
36
The categories of this paper are: cmp-lg
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th