  Based on an approach to non-Abelian propagation of color dipoles in a nuclear
medium we formulate a nonlinear k_t-factorization for the breakup of photons
and pions into forward hard dijets in terms of the collective
Weizs\"acker-Williams (WW) glue of nuclei. We find quite distinct practical
consequences of nonlinear nuclear k_t-factorization for interactions of
pointlike photons and non-pointlike pions. In the former case the large
transverse momentum p_t of jets comes from the intrinsic momentum of quarks and
antiquarks in the photon and nuclear effects manifest themselves as an
azimuthal decorrelation with an acoplanarity momentum of the order of the
nuclear saturation momentum Q_A. In the breakup of pions off free nucleons to
the leading order in pQCD the spectator parton has a small transverse momentum
and the hard dijet cross section is suppressed. In the breakup of pions off
heavy nuclei the forward hard jets are predicted to be entirely decorrelated.
We comment on the sensitivity of the pionic dijet cross section to the pion
distribution amplitude. The predicted distinction between the breakup of
photons and pions can be tested by the sphericity and thrust analysis of the
forward hadronic system in the COMPASS experiment at CERN.
