We study the transverse single spin asymmetry in direct photon production in
pA collisions with incoming protons being transversely polarized. To facilitate
the calculation, we formulate a hybrid approach in which the nucleus is treated
in the Color Glass Condensate (CGC) framework while the collinear twist-3
formalism is applied on the proton side. It has been found that an additional
term which arises from color entanglement shows up in the spin dependent
differential cross section. The fact that this additional term is
perturbatively calculable allows us to quantitatively study color entanglement
effects.Comment: 20 pages, 6 figures, a few typos corrected, v
