Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Highest Energy Cosmic Rays, Gamma Rays and Neutrinos: Facts, Fancy
  and Resolution   Although cosmic rays were discovered 90 years ago, we do not know how and
where they are accelerated. There is compelling evidence that the highest
energy cosmic rays are extra-galactic -- they cannot be contained by our
galaxy's magnetic field anyway because their gyroradius exceeds its dimensions.
Elementary elementary-particle physics dictates a universal upper limit on
their energy of $5\times10^{19}$ eV, the so-called Greisen-Kuzmin-Zatsepin
cutoff; however, particles in excess of this energy have been observed, adding
one more puzzle to the cosmic ray mystery. Mystery is nonetheless fertile
ground for progress: we will review the facts and mention some very speculative
interpretations. There is indeed a realistic hope that the oldest problem in
astronomy will be resolved soon by ambitious experimentation: air shower arrays
of $10^4$ km$^2$ area, arrays of air Cerenkov detectors and kilometer-scale
neutrino observatories.

10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
21
The categories of this paper are: adap-org
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg