Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Limit on the pion distribution amplitude   The pion distribution amplitude (DA) can be related to the fundamental QCD
Green's functions as a function of the quark self-energy and the quark-pion
vertex, which in turn are associated with the pion wave function through the
Bethe-Salpeter equation. Considering the extreme hard asymptotic behavior in
momentum space allowed for a pseudoscalar wave function, which is limited by
its normalization condition, we compute the pion DA and its second moment. From
the resulting amplitude, representing the field theoretical upper limit on the
DA behavior, we calculate the photon-pion transition form factor
$F_{\pi\gamma\gamma^{\ast}}(Q^{2})$. The resulting upper limit on the pion
transition form factor is compared with existing data published by CLEO, BaBar
and Belle collaborations.

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The categories of this paper are: math
36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
22
The categories of this paper are: chao-dyn
3
The categories of this paper are: physics
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
21
The categories of this paper are: adap-org
30
The categories of this paper are: q-alg
10
The categories of this paper are: q-bio