Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The number of colors in the decays pi^0, eta, eta-prime -> gamma gamma   The decays pi^0, eta, eta-prime -> gamma gamma are investigated up to
next-to-next-to-leading order in the framework of the combined 1/N_c and chiral
expansions. Without mixing of the pseudoscalar mesons the N_c independence of
the pi^0 and eta decay amplitudes is shown to persist at the one-loop level,
although the contribution of the Wess-Zumino-Witten term to the pertinent
vertices is not canceled by the N_c dependent part of a Goldstone-Wilczek term.
The decay amplitude of the singlet field, on the other hand, depends strongly
on N_c and yields under the inclusion of mixing also a strong N_c dependence
for the eta decay. Both the eta and eta-prime decay are suited to confirm the
number of colors to be N_c=3.

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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: chem-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: stat
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The categories of this paper are: hep-ex
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The categories of this paper are: cond-mat
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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: q-alg
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The categories of this paper are: nlin
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The categories of this paper are: q-fin
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The categories of this paper are: plasm-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: mtrl-th