Associated forward photon-jet production as a probe of the nuclear unintegrated glue

To the lowest order in pQCD, the underlying subprocess for hard isolated photon production in the proton fragmentation region of pp and pA collisions is the QCD Compton effect q * g → qγ, where q * is the valence quark in the beam proton.In the q * p collisions frame, the qg dijet acoplanarity momentum ∆ comes from the exchanged gluon and the ∆-distribution measures the unintegrated gluon density in the target nucleon.Both the incident and produced quarks undergo multiple gluon exchanges in an extended nuclear target; in all other dijet processes that entails a complex nonlinear k ⊥ -factorization for the dijet spectrum.We demonstrate that the qγ dijet production is an exceptional reaction for which the linear k ⊥ -factorization holds, which makes it a unique direct probe of the collective nuclear gluon density at small-x.We discuss properties of the nuclear unintegrated gluon distribution, and derive a relationship between the nuclear broadening of the acoplanarity momentum distribution and the nuclear saturation scale.

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