Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at √s_NN =5.02 TeV

The pseudorapidity density of charged particles, dN_ch/dη, at midrapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of √s_NN = 5.02  TeV. For the 5% most central collisions, we measure a value of 1943±54. The rise in dN_ch/dη as a function of √s_NN is steeper than that observed in proton-proton collisions and follows the trend established by measurements at lower energy. The increase of dN_ch/dη as a function of the average number of participant nucleons, ⟨Npart⟩, calculated in a Glauber model, is compared with the previous measurement at √s_NN = 2.76  TeV. A constant factor of about 1.2 describes the increase in dN_ch/dη from √s_NN = 2.76 to 5.02 TeV for all centrality classes, within the measured range of 0%–80% centrality. The results are also compared to models based on different mechanisms for particle production in nuclear collisions.