Universality of unintegrated gluon distributions at small<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>x</mml:mi></mml:math>

We systematically study dijet production in various processes in the small-$x$ limit and establish an effective ${k}_{t}$-factorization for hard processes in a system with dilute probes scattering on a dense target. We find that the well-known Weizs\"acker-Williams gluon distribution can be directly probed in the quark-antiquark jet correlation in deep inelastic scattering and the dipole gluon distribution can be directly measured in the direct photon-jet correlation in $pA$ collisions. In the large-${N}_{c}$ limit, the unintegrated gluon distributions involved in other different dijet channels in $pA$ collisions are shown to be related to two widely proposed ones: the Weizs\"acker-Williams gluon distribution and the dipole gluon distribution.

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