Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$

The pseudorapidity density of charged particles, $d{N}_{\mathrm{ch}}/d\eta$, at midrapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. For the 5% most central collisions, we measure a value of $1943\pm 54$. The rise in $d{N}_{\mathrm{ch}}/d\eta$ as a function of $\sqrt{s_{NN}}$ is steeper than that observed in proton-proton collisions and follows the trend established by measurements at lower energy. The increase of $d{N}_{\mathrm{ch}}/d\eta$ as a function of the average number of participant nucleons, $⟨N_{\text{part}}⟩$, calculated in a Glauber model, is compared with the previous measurement at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. A constant factor of about 1.2 describes the increase in $d{N}_{\mathrm{ch}}/d\eta$ from $\sqrt{s_{NN}}=2.76$ 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.

Figure 1

Values of $(2/⟨N_{\mathrm{part}}⟩)⟨d{N}_{\mathrm{ch}}/d\eta ⟩$ for central Pb-Pb [4, 5, 6, 7] and Au-Au [8, 9, 10, 11, 12] collisions (see the text) as a function of $\sqrt{s_{NN}}$. Measurements for inelastic $pp$ and $p\overline{p}$ collisions as a function of $\sqrt{s}$ are also shown [26, 27, 28] along with those from nonsingle diffractive $p\text{−}A$ and $d\text{−}A$ collisions [29, 30]. The $s$ dependencies of the $AA$ and $pp$ ($p\overline{p}$) collision data are well described by the functions $s_{NN}^{0.155}$ (solid line) and $s_{NN}^{0.103}$ (dashed line), respectively. The shaded bands show the uncertainties on the extracted power-law dependencies. The central Pb-Pb measurements from CMS and ATLAS at 2.76 TeV have been shifted horizontally for clarity.

Figure 2

The $(2/⟨N_{\mathrm{part}}⟩)⟨d{N}_{\mathrm{ch}}/d\eta ⟩$ for Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in the centrality range 0%–80%, as a function of $⟨N_{\mathrm{part}}⟩$ in each centrality class. The error bars indicate the point-to-point centrality-dependent uncertainties, whereas the shaded band shows the correlated contributions. Also shown is the result from nonsingle diffractive $p$-Pb collisions at the same $\sqrt{s_{NN}}$ [18]. Data from lower-energy (2.76 TeV) Pb-Pb and $pp$ collisions [4, 26], scaled by a factor of 1.2 and 1.13, respectively, are shown for comparison. The error bars for $p$-Pb at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ and lower-energy Pb-Pb and $pp$ collisions indicate the total uncertainty.

Figure 3

The $(2/⟨N_{\mathrm{part}}⟩)⟨d{N}_{\mathrm{ch}}/d\eta ⟩$ for Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in the centrality range 0%–80%, as a function of $⟨N_{\mathrm{part}}⟩$ in each centrality class, compared to model predictions [31, 32, 33, 34, 35, 36, 37, 38, 39].