Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Physics Case for Polarised Proton-Nucleon Scattering at HERA   The physics case for a possible fixed target polarized nucleon-nucleon
collision experiment at HERA is described. The experiment named HERA-N could be
realized using an internal polarized gas target in the HERA
polarized/unpolarized proton beam. A wide spectrum of nucleon spin structure
problems could be investigated and the experiment would constitute a fixed
target complement to the RHIC spin physics program with competitive statistical
accuracy.

22
The categories of this paper are: chao-dyn
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The categories of this paper are: math
5
The categories of this paper are: gr-qc
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The categories of this paper are: eess
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The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
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The categories of this paper are: stat
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The categories of this paper are: nucl-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: solv-int
4
The categories of this paper are: cond-mat
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The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga