Photon from the annihilation process with CGC in the $pA$ collision

Understand

We discuss the photon production in the $pA$ collision in a framework of the color glass condensate (CGC) with expansion in terms of the proton color source $\rho_p$.

• We work in a regime where the color density $\rho_A$ of the nucleus is large enough to justify the CGC treatment, while soft gluons in the proton could be dominant over quark components but do not yet belong to the CGC regime, so that we can still expand the amplitude in powers of $\rho_p$.
• The zeroth-order contribution to the photon production is known to appear from the Bremsstrahlung process and the first-order corrections consist of the Bremsstrahlung diagrams with pair produced quarks and the annihilation diagrams of quarks involving a gluon sourced by $\rho_p$.
• Because the final states are different there is no interference between these two processes.