Abstract
The centrality dependence of the charged-particle pseudorapidity density measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ over a broad pseudorapidity range is presented. This Letter extends the previous results reported by ALICE to more peripheral collisions. No strong change of the charged-particle pseudorapidity density distributions with centrality is observed, and when normalised to the number of participating nucleons in the collisions, the evolution over pseudorapidity with centrality is likewise small. The broad pseudorapidity range allows precise estimates of the total number of produced charged particles which we find to range from $162\pm22$ (syst.) to $17170\pm770$ (syst.) in 80-90% and 0-5 central collisions, respectively. The total charged-particle multiplicity is seen to approximately scale with the number of participating nucleons in the collision. This suggests that hard contributions to the charged-particle multiplicity are limited. The results are compared to models which describe $\mbox{d}N_{\mbox{ch}}/\mbox{d}\eta$ at mid-rapidity in the most central Pb-Pb collisions and it is found that these models do not capture all features of the distributions.
Highlights
The measurement of the charged-particle pseudorapidity (η) density distribution in heavy-ion collisions provides insight into the dominant particle production mechanisms, such as parton fragmentation [1] and the observed phenomenon of limiting fragmentation [2]
The unique capability of ALICE to perform such measurements from large to small overlaps of the colliding nuclei over a broad pseudorapidity range allows for significant additional information to be extracted e.g., the total number of charged particles and the evolution of the distributions with centrality
The ATLAS Collaboration has reported on the charged-particle pseudorapidity density in the 80% most central events in a limited pseudorapidity range of |η| < 2 [8]
Summary
The measurement of the charged-particle pseudorapidity (η) density distribution in heavy-ion collisions provides insight into the dominant particle production mechanisms, such as parton fragmentation [1] and the observed phenomenon of limiting fragmentation [2]. Various models [3,4,5] make different assumptions on how particles are produced in heavy-ion collisions resulting in very different charged-particle pseudorapidity density distributions — both in terms of scale and shape. The ALICE Collaboration has previously reported results on the charged-particle Pb–Pb collisions pasteu√dsoNrNap=id2it.y76deTnesVityovienr the 0–30% most central a wide pseudorapidity range [6], and in the 80% most central collisions at mid-rapidity (η ≈ 0) only [7]. The ATLAS Collaboration has reported on the charged-particle pseudorapidity density in the 80% most central events in a limited pseudorapidity range of |η| < 2 [8].
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