Color entanglement for $\gamma$-jet production in polarized $pA$ collisions

A more reliable treatment of transverse momentum-dependent physics and in particular transverse single-spin asymmetries is urgently required, e.g. for polarized $pA$ physics including novel effects like color entanglement. We argue that the measurement of azimuthal angular correlations of photons and jets produced in $pA$ collisions provides a direct access to the novel gluon distribution $G_4$ that enters into many such processes, though practically it will be very challenging to probe this distribution experimentally as it is a $1/N_c^2$-suppressed effect.

Figure 1

Diagrams contributing to the spin-dependent photon-jet production amplitude. Different symbols indicate different parts of the classical gluon field. A black dot denotes $A_{\text{reg}}$ or $A_{\text{sing}}$, while a cross surrounded with a circle denotes $A_A$. All diagrams from (a)–(p) contribute to the initial state interaction, while only diagrams (b) and (d) also give rise to the final state interaction contribution.