Heavy-ion collisions at the LHC

A new era in the study of high-energy nuclear collisions began when the CERN Large Hadron Collider (LHC) provided the first collisions of lead nuclei in late 2010. In the first three years of operation the ALICE, ATLAS and CMS experiments each collected Pb–Pb data samples of more than 50μb−1  at sNN=2.76TeV, exceeding the previously studied collision energies by more than an order of magnitude. These data have provided new insights into the properties of QCD matter under extreme conditions, with extensive measurements of soft particle production and newly accessible hard probes of the hot and dense medium. In this review, we provide a comprehensive overview of the results obtained in heavy-ion collisions at the LHC so far, with particular emphasis on the complementary nature of the observations by the three experiments. In particular, the combination of ALICE’s strengths at hadron identification, the strengths of ATLAS and CMS to make precise measurements of high pT  probes, and the resourceful measurements of collective flow by all of the experiments have provided a rich and diverse dataset in only a few years. While the basic paradigm established at RHIC — that of a hot, dense medium that flows with a viscosity to shear-entropy ratio near the predicted lower bound, and which degrades the energy of probes, such as jets, heavy-flavours and J/ψ — is confirmed at the LHC, the new data suggest many new avenues for extracting its properties in detail.