The 28th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, also known as “Quark Matter”, took place in Wuhan, China, in November. More than 800 participants discussed the latest results of the heavy-ion programmes at the Large Hadron Collider and at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC), as well as the most recent theoretical developments. The focus of these studies is the fundamental understanding of strongly interacting matter at extremes of temperature and density. In these conditions, which also characterise the early universe, matter is a quark-gluon plasma (QGP), in which quarks and gluons are not confined within hadrons. In the recent editions of Quark Matter, much attention has also been devoted to the study of emergent QCD phenomena in high-multiplicity proton-proton and proton-nucleus collisions, which resemble the collective effects seen in nucleus-nucleus collisions and pose the intriguing question of whether a QGP can also form in “small-system” collisions.
The LHC and RHIC together cover a broad range of quark-gluon-plasma temperatures
The large data sample from the Pb-Pb period of LHC Run 2 in 2018 allowed ALICE, ATLAS, CMS and LHCb to study rare probes of the QGP, such as jets and heavy quarks, with unprecedented precision. New constraints on the energy loss of partons when traversing the high-density medium were presented, pushing the limits of jet measurements to lower transverse momenta and larger radii: jet modifications are now measured in the transverse momentum range from 40 to 1000 GeV/c and in the jet radius (resolution parameter) range 0.2 to 1. The internal structure of jets was studied not only by the LHC experiments, but also by the PHENIX and STAR collaborations at the 25-times lower RHIC collision energy. LHC and RHIC measurements are complementary as they cover a broad range of QGP temperatures and differ in the balance of quark- and gluon-initiated jets, with the former dominating at RHIC and the latter dominating at the LHC.
Measurements in the sectors of heavy quarks and rarely-produced light nuclei (such as deuterons, 3He and hypertriton, a pnΛ bound state) also strongly benefitted from the large recent samples recorded at the LHC. In particular, their degree of collective behaviour could be studied in much greater detail. The family of QGP probes in the heavy-quark sector has been extended with new members at the LHC by first observations of the X(3872) exotic hadron and of top-antitop quark production. In the sector of electromagnetic processes, new experimental observations were presented for the first time at the conference, including the photo-production of dileptons in collisions with and without hadronic overlap, and light-by-light scattering. These effects are induced by the interaction of the strong electromagnetic fields of the two Pb nuclei (Z=82) passing close to each other (CERN Courier January/February 2020, p17).
In nuclear collisions the fluid-dynamical flow of the QGP leaves an imprint in the azimuthal distribution of soft particles, as the initial geometry of the collision is translated to flow through pressure gradients. Its experimental trace is multi-particle angular correlations between low-momentum particles, even at large rapidity separations. In non-central nucleus-nucleus collisions that have an elliptical initial geometry, the resulting azimuthal modulation of particles momentum distribution is called elliptic flow. New information on collective behaviour and on the dynamics of heavy-quark interactions in the QGP was added by a first measurement of the D-meson momentum distribution down to zero momentum in Pb-Pb collisions at the LHC, and by new measurements of the elliptic flow of D mesons, muons from charm and beauty decays as well as bound states of heavy quarks (charmonia and bottomonia). These measurements suggest a stronger degree of collective behaviour for light than heavy quarks, and further constrain estimates of the QGP viscosity. Such estimates also require understanding of heavy-quark hadronisation, which was discussed in the light of new results at RHIC and the LHC which indicate an increased production of charmed baryons with respect to mesons, at low momentum in both pp and nucleus-nucleus collisions, when compared to expectations from electron-positron collisions.
The situation is much less clear in the collisions of small systems
While there is strong evidence for the production of QGP in nuclear collisions, the situation is much less clear in the collisions of small systems. The momentum correlations and azimuthal modulation that characterise the large nuclear collisions were also observed in smaller collision systems, such as p-Pb at the LHC, p-Au, d-Au and 3He-Au at RHIC, and even pp. The persistence of these correlations in smaller collision systems, down to pp collisions where it is unlikely that an equilibrated system could be created, may offer an inroad to understand how the collective behaviour of the QGP arises from the microscopic interaction of its individual constituents. New measurements on multi-particle correlations were presented and the dynamical origin of the collectivity in small systems was discussed. Small expanding QGP droplets, colour connections of overlapping QCD strings, and final-state rescattering at partonic or hadronic level are among the possible mechanisms that are proposed to describe these observations. While many signs characteristic of the QGP are seen in the small-system collisions, parton energy loss (in the form of jet or large-momentum hadron modifications) remains absent in the measurements carried out to date.
Beyond Quark Matter 2019, the field is now looking forward to the future programmes at the LHC and at RHIC, which were extensively reviewed at the conference. At the LHC, the heavy-ion injectors and the experiments are currently being upgraded. In particular, the heavy-ion-dedicated ALICE detector is undergoing major improvements, with readout and tracker upgrades that will provide larger samples and better performance for heavy-flavour selection. Run 3 of the LHC, which is scheduled to start in 2021, will provide integrated luminosity increases ranging from one order of magnitude for the data samples based on rare triggers to two orders of magnitude for the minimum-bias (non-triggered) samples. At RHIC, the second beam-energy-scan programme is now providing the STAR experiment with higher precision data to search for the energy evolution of QGP effects, and the new sPHENIX experiment aims at improved measurements of jets and heavy quarks from 2023. Low-energy programmes at the CERN SPS, NICA, FAIR, HIAF and J-PARC, which target a systematic exploration of heavy-ion collisions with high baryon density to search for the onset of deconfinement and the predicted QCD critical point, were also discussed in Wuhan, and the updated plans for the US-based Electron-Ion Collider (EIC), which is foreseen to be constructed at Brookhaven National Laboratory, were presented. With ep and e-nucleus interactions, the EIC will provide unprecedented insights into the structure of the proton and the modification of parton densities in nuclei, which will benefit our understanding of the initial conditions for nucleus-nucleus collisions.