The Color Glass Condensate (CGC) effective theory and the collinear
factorization at high-twist (HT) are two well-known frameworks describing
perturbative QCD multiple scatterings in nuclear media. It has long been
recognized that these two formalisms have their own domain of validity in
different kinematics regions. Taking direct photon production in proton-nucleus
collisions as an example, we clarify for the first time the relation between
CGC and HT at the level of a physical observable. We show that the CGC
formalism beyond shock-wave approximation, and with the
Landau-Pomeranchuk-Migdal interference effect is consistent with the HT
formalism in the transition region where they overlap. Such a unified picture
paves the way for mapping out the phase diagram of parton density in nuclear
medium from dilute to dense region.Comment: 7 pages, 3 figures + supplemental materia