Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Past, Present and Future of UHECR Observations   Great advances have been made in the study of ultra-high energy cosmic rays
(UHECR) in the past two decades. These include the discovery of the spectral
cut-off near 5 x 10^19 eV and complex structure at lower energies, as well as
increasingly precise information about the composition of cosmic rays as a
function of energy. Important improvements in techniques, including extensive
surface detector arrays and high resolution air fluorescence detectors, have
been instrumental in facilitating this progress. We discuss the status of the
field, including the open questions about the nature of spectral structure,
systematic issues related to our understanding of composition, and emerging
evidence for anisotropy at the highest energies. We review prospects for
upgraded and future observatories including Telescope Array, Pierre Auger and
JEM-EUSO and other space-based proposals, and discuss promising new
technologies based on radio emission from extensive air showers produced by
UHECR.

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The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
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The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: patt-sol
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The categories of this paper are: adap-org
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-lat
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The categories of this paper are: eess
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The categories of this paper are: plasm-ph