System size, energy, centrality and pseudorapidity dependence of charged-particle density in Au+Au and Cu+Cu collisions at RHIC

TL;DR

This study measures charged-particle pseudorapidity distributions in Au+Au and Cu+Cu collisions at various energies and centralities, revealing that the shape and total number of produced particles depend primarily on the nuclear overlap geometry rather than just participant count.

Contribution

It demonstrates that the detailed shape of pseudorapidity distributions is governed by the geometry of the nuclear overlap zone, not solely by the number of nucleon participants.

Findings

Total charged particle production correlates with the number of nucleon participants.

Shape of pseudorapidity distributions depends on the nuclear overlap geometry.

Matching distribution shapes is better when considering N_part/2A than N_part alone.

Abstract

Charged particle pseudorapidity distributions are presented from the PHOBOS experiment at RHIC, measured in Au+Au and Cu+Cu collisions at sqrt{s_NN}=19.6, 22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The measurements were made by the same detector setup over a broad range in pseudorapidity, |eta|<5.4, allowing for a reliable systematic study of particle production as a function of energy, centrality and system size. Comparing Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the overall shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants, N_part. Detailed comparisons reveal that the matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of eta is better for the same N_part/2A value than for the same N_part value, where A denotes the mass number. In other words, it is the geometry of the nuclear overlap zone, rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence.