Hidden charm in the quark–gluon plasma

For almost 40 years, charmonium, a bound state of a heavy charm–anticharm pair (hence also called a hidden charm), has provided a unique probe to study the properties of the quark–gluon plasma (QGP), the state of matter composed by deconfined quarks and gluons present in the early instants of the universe and produced experimentally in ultrarelativistic heavy-ion collisions. Charmonia come in a rich variety of states. In a new analysis investigating how these different bound charmonium states are affected by the QGP, the ALICE collaboration has opened a novel way to study the strong interaction at extreme temperatures and densities. 

In the QGP, the production of charmonium is suppressed due to “colour screening” by the large number of quarks and gluons present. The screening, and thus the suppression, increases with the temperature of the QGP and is expected to affect different charmonium states to different degrees. The production of the ψ(2S) state, for example, which is 10 times more weakly bound and two times larger in size than the most tightly bound state, the J/ψ, is expected to be more suppressed. 

This hierarchical suppression is not the only fate of charmonia in the quark–gluon plasma. The large number of charm quarks and antiquarks in the plasma – up to about 100 in head-on lead–lead collisions – also gives rise to a mechanism, called recombination, that forms new charmonia and counters the suppression to a certain extent. This process is expected to depend on the type and momentum of the charmonia, with the more weakly bound charmonia being produced through recombination later in the evolution of the plasma and charmonia with the lowest (transverse) momentum having the highest recombination rate. 

Previous studies, using data first from the Super Proton Synchrotron and then from the LHC, have shown that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ, and ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. But so far, no studies of ψ(2S) production at low transverse particle momentum had been precise enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.

The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018. The results indicate that the ψ(2S) yield is largely suppressed with respect to a proton–proton baseline, almost a factor of two more suppressed than the J/ψ. The suppression, shown as a function of the collision centrality (N_part) in the figure, is quantified through the nuclear modification factor (R_AA), which compares the particle production in lead–lead collisions with respect to the expectations based on proton–proton collisions.  

Theoretical predictions based on a transport approach that includes suppression and recombination of charmonia in the QGP (TAMU) or on the Statistical Hadronisation Model (SHMc), which assumes charmonia to be formed only at hadronisation, describe the J/ψ data, while the ψ(2S) production is underestimated in central events by the SHMc. This observation represents one of the first indications that dynamical effects in the QGP, as taken into account in the transport models, are needed to reproduce the yields of the various charmonium states. It also shows that precision studies, including these and those of other charmonia, and foreseen for Run 3 of the LHC, may lead to a final understanding of the modification of the force binding these states in the extreme environment of the QGP.