Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Probing the gluon tomography in photoproduction of di-pions   A sizable $\cos 4\phi$ azimuthal asymmetry in exclusive di-pion production
near $\rho^0$ resonance peak in ultraperipheral heavy ion collisions recently
has been reported by STAR collaboration. We show that both elliptic gluon
Wigner distribution and final state soft photon radiation can give rise to this
azimuthal asymmetry. The fact that the QED effect alone severely underestimates
the observed asymmetry might signal the existence of the nontrivial correlation
in quantum phase distribution of gluons.

14
The categories of this paper are: math-ph
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
17
The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
27
The categories of this paper are: comp-gas
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex