Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Searching for the rules that govern hadron construction   Just as Quantum Electrodynamics describes how electrons are bound in atoms by
the electromagnetic force, mediated by exchange of photons, Quantum
Chromodynamics (QCD) describes how quarks are bound inside hadrons by the
strong force, mediated by exchange of gluons. At face value, QCD allows hadrons
constructed from increasingly many quarks to exist, just as atoms with
increasing numbers of electrons exist, yet such complex constructions seemed,
until recently, to not be present in nature. In what follows we describe
advances in the spectroscopy of mesons that are refining our understanding of
the rules for building hadrons from QCD.

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The categories of this paper are: dg-ga
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The categories of this paper are: astro-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: atom-ph
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The categories of this paper are: supr-con
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The categories of this paper are: math
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The categories of this paper are: stat
3
The categories of this paper are: physics
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The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: ao-sci
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The categories of this paper are: math-ph
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The categories of this paper are: hep-th