Centrality dependence of the pseudorapidity density distribution for charged particles in Pb–Pb collisions at [formula omitted

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doi:10.1016/j.physletb.2017.07.017

Abstract

We present the charged-particle pseudorapidity density in Pb–Pb collisions at sNN=5.02 TeV 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 21400±1300, while for the most peripheral (80–90%) we find 230±38. This corresponds to an increase of (27±4)% over the results at sNN=2.76 TeV 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.

Highlights

In ultra-relativistic heavy-ion collisions a dense and hot phase of nuclear matter is created [1,2,3,4]
The measurements from the Silicon Pixel Detector (SPD) and Forward Multiplicity Detector (FMD) are combined in regions of overlap (1.8 < |η| < 2) between the two detectors by taking the weighted average using the non-shared uncertainties as weights
The total number of charged particles produced is determined owing to the large pseudorapidity acceptance of AL

Summary

Introduction

In ultra-relativistic heavy-ion collisions a dense and hot phase of nuclear matter is created [1,2,3,4]. This phase of QCD matter is considered to be a plasma of strongly interacting quarks and gluons and is labelled the sQGP [5]. The study of the primary charged-particle pseudorapidity density (dNch/dη) over a wide pseudorapidity (η) range and its dependence on colliding system, centre-of-mass energy, and collision geometry is important to understand the relative contributions to particle production from hard scatterings and soft processes, and may provide insight into the partonic structure of the interacting nuclei. Their collisions can be characterised by centrality — the experimental proxy for the un-measurable distance between the centres of the colliding nuclei (impact parameter)

Methods

Results

Conclusion