Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultrahigh Energy Cosmic Rays   This is a review of the most resent results from the investigation of the
Ultrahigh Energy Cosmic Rays, particles of energy exceeding 10$^{18}$ eV. After
a general introduction to the topic and a brief review of the lower energy
cosmic rays and the detection methods, the two most recent experiments, the
High Resolution Fly's Eye (HiRes) and the Southern Auger Observatory are
described. We then concentrate on the results from these two experiments on the
cosmic ray energy spectrum, the chemical composition of these cosmic rays and
on the searches for their sources. We conclude with a brief analysis of the
controversies in these results and the projects in development and construction
that can help solve the remaining problems with these particles.

19
The categories of this paper are: eess
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
23
The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
30
The categories of this paper are: q-alg
20
The categories of this paper are: acc-phys