Nonlinear $k_\perp$-factorization: a new paradigm for hard QCD processes in a nuclear environment

Large thickness of heavy nuclei brings in a new hard scale into the pQCD description of hard processes in a nuclear environment. This new scale breaks the conventional linear $k_\perp$-factorization which must be replaced by a new concept of the nonlinear nuclear $k_\perp$-factorization. Here we review a recent progress in the theory of nonlinear $k_\perp$-factorization. Our focus is on the rôle of diffractive interactions, the variation of the pattern of $k_\perp$-factorization for single-jet processes from deep inelastic scattering to hadron-nucleus collisions and universality classes for dijet production off nuclei.