Centrality dependence of the pseudorapidity density distribution for charged particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

TL;DR

This paper measures the distribution of charged particles produced in Pb-Pb collisions at 5.02 TeV, revealing energy-dependent increases and a proportionality between rapidity distribution width and beam rapidity, with implications for collision modeling.

Contribution

It provides the first detailed pseudorapidity density distribution over a wide range at 5.02 TeV and compares results with models, highlighting discrepancies and energy dependence.

Findings

Charged-particle multiplicity increases by 27% from 2.76 TeV to 5.02 TeV.

Distribution width scales proportionally with beam rapidity across energies.

Models do not fully reproduce the measured distributions.

Abstract

We present the charged-particle pseudorapidity density in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{Te\kern-.25exV}$ in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from $-3.5$ to $5$, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find $21\,400\pm 1\,300$ while for the most peripheral (80-90%) we find $230\pm 38$. This corresponds to an increase of $(27\pm4)\%$ over the results at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{Te\kern-.25exV}$ previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations --- none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.