High-energy collisions of lead nuclei at the Large Hadron Collider (LHC) explore strongly-interacting matter under the most extreme conditions on Earth. These heavy-ion collisions recreate the quark-gluon plasma (QGP): the hottest and densest fluid, in which quarks and gluons are not confined within hadrons.
The ALICE experiment was designed to study the QGP at LHC energies. It has been operated during the LHC Runs 1 and 2, and has carried out a broad range of measurements to characterize the QGP and to study several other aspects of the strong interaction. In a recent review the ALICE collaboration takes stock of its first decade of studies related to QCD. Quantum chromodynamics (QCD) is the theory of the strong interaction. The results from the ALICE studies include a suite of observables that reveal a complex evolution of the near-perfect QGP liquid that emerges in high-temperature QCD. ALICE measurements also demonstrate that charm quarks equilibrate extremely quickly within this liquid, and are able to regenerate QGP-melted charmonium states. ALICE has extensively mapped the QGP opacity with high-energy probes, and directly observed the QCD dead-cone effect in pp collisions. Surprising QGP-like effects have also been observed in rare pp and p-Pb collisions. Finally, ALICE measurements of interactions of produced hadrons have also revealed novel features that have broad implications for nuclear and astrophysics.
Further reading: ALICE collaboration, arXiv:2211.04384