Heavy quarkonium production: Nontrivial transition from pA to AA collisions

Understand

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.