  Two novel QCD effects, double color filtering and mutual boosting of the
saturation scales in colliding nuclei, affect the transparency of the nuclei
for quark dipoles in comparison with proton-nucleus collisions. The former
effect increases the survival probability of the dipoles, since color filtering
in one nucleus makes the other one more transparent. The second effect acts in
the opposite direction and is stronger, it makes the colliding nuclei more
opaque than in the case of pA collisions. As a result of parton saturation in
nuclei the effective scale is shifted upwards, what leads to an increase of the
gluon density at small x. This in turn leads to a stronger transverse momentum
broadening in AA compared with pA collisions, i.e. to an additional growth of
the saturation momentum. Such a mutual boosting leads to a system of
reciprocity equations, which result in a saturation scale, a few times higher
in AA than in pA collisions at the energies of LHC. Since the dipole cross
section is proportional to the saturation momentum squared, the nuclei become
much more opaque for dipoles in AA than in pA collisions. For the same reason
gluon shadowing turns out to be boosted to a larger magnitude compared with the
product of the gluon shadowing factors in each of the colliding nuclei. All
these effects make it more difficult to establish a baseline for anomalous
J/Psi suppression in heavy ion collisions at high energies.
