Quarkonium states, such as the J/ψ meson, are prominent probes of the quark–gluon plasma (QGP) formed in high-energy nucleus–nucleus (AA) collisions. That bulk J/ψ production is suppressed in AA collisions with respect to proton–proton collisions had been reported by ALICE five years ago. However, measurements of J/ψ production in proton–lead collisions, where the formation of the QGP is not expected, are essential to quantify effects that are present in AA collisions but not associated with the QGP. In a recent study, ALICE has shown that the production of J/ψ mesons in proton–lead collisions is strongly correlated with the total number of produced particles in the event (event multiplicity), and that this correlation varies as a function of rapidity.

ALICE

In ALICE, the J/ψ measurements are performed at forward (proton direction), mid- and at backward-rapidity (lead direction). An increase of the J/ψ yield relative to the event-averaged value with the relative charged-particle multiplicity is observed for all rapidity domains, with a similar slope at low multiplicities (see figure). At multiplicities a factor two above the event average, the trend at forward rapidity is very different from those at mid- and backward-rapidity. In the forward rapidity window, a saturation of the relative yield sets in at high multiplicities, which is interesting because the forward region with low parton fractional momentum is in the domain of gluon shadowing/saturation.

Models incorporating nuclear parton distribution functions with significant shadowing have previously been shown to describe J/ψ measurements performed in event classes selected according to the centrality of the collision. The present measurement, exploring significantly more “violent” events (below 1% of the total hadronic interaction cross-section), suggests that effective gluon depletion in the colliding lead nucleus is larger in high-multiplicity events. However, there are additional concepts to describe this regime of QCD, and it remains to be seen whether such models can also describe the saturation of the yields at forward rapidities.