Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The highest energy cosmic rays: the past, the present and the future   The greater part of this paper is concerned with a historical discussion of
the development of the search for the origins of the highest-energy cosmic-rays
together with a few remarks about future prospects.
  Additionally, in section 6, the situation with regard to the mass composition
and energy spectrum at the highest energies is discussed. It is shown that the
change of the depth of shower maximum with energy above 1 EeV, measured using
the Telescope Array, is in striking agreement with similar results from the
Auger Observatory. This implies that either the mean mass of cosmic rays is
becoming heavier above ~4 EeV or that there is a change in details of the
hadronic interactions in a manner such that protons masquerade as heavier
nuclei. A long-standing controversy is thus resolved: the belief that pure
protons dominate the mass distribution at the highest energies is no longer
tenable.

15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
11
The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph