After the restart of the LHC physics programme in June 2015 with world-record proton– proton collisions at √s = 13 TeV, nuclear beams reappeared in the LHC tunnel in November 2015 with subsequent first collisions between 208Pb ions. With unprecedented centre-of-mass energy values of 5.02 TeV in the nucleon–nucleon system, collection of these data marks the beginning of a new chapter in the precision study of properties of hot and dense hadronic matter, and the quest to understand QCD confinement.
Measurement of the inclusive production of charged hadrons in high-energy nucleus– nucleus reactions is a key observable to characterise the global properties of the collision, in particular, whenever the collision energy increases significantly (almost a factor of two with respect to the LHC Run 1). Particle production at collider energies originates from the interplay of perturbative (hard) and non-perturbative (soft) QCD processes. Soft scattering processes and parton hadronisation dominate the bulk of particle production at low transverse momenta, and can only be modelled phenomenologically. On the other hand, with an increase in collision energy, the role of hard processes – parton scatterings with large momentum transfer – increases. Such measurements, which contribute essential information to estimate the initial energy density leading to the formation and evolution of the quark–gluon plasma and its relation to the collision geometry, also provide valuable insight into the initial-state partonic structure of the colliding nuclei.
The ALICE experiment has measured the centrality-dependence of the inclusive charged-particle density (dNch/dη) at mid-rapidity (|η| < 0.5) in Pb–Pb collisions at √sNN = 5.02 TeV. For an event sample corresponding to the most central 5% of the hadronic cross-section, the pseudorapidity density of primary charged particles at midrapidity is 1943±54, which corresponds to 10.2±0.3 per participating nucleon pair. This represents an increase of a factor of about 2.4 relative to p–Pb collisions at the same collision energy, and a factor of about 1.2 to central Pb–Pb collisions at 2.76 TeV. Previous measurements were performed by ALICE, ATLAS and CMS at the LHC at √sNN = 2.76 TeV, and also at lower energies in the range √sNN = 17–200 GeV with SPS and RHIC experiments. The figure shows a compilation of results on mid-rapidity charged-particle density for most central nucleus–nucleus collisions and elementary proton–proton and proton(deuteron)–nucleus collisions. Particle production in nucleus–nucleus collisions increases more rapidly with the centre-of-mass energy (per nucleon pair) than in proton–proton and proton(deuteron)–nucleus collisions, in agreement with expectations from the power-law extrapolation of lower-energy results. The characteristics of the centrality dependence of dNch/dη and comparison with several phenomenological models is reported in a recent publication by the ALICE collaboration.