Universality of unintegrated gluon distributions at small x

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 Weizsäcker-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 Weizsäcker- Williams gluon distribution and the dipole gluon distribution.