System Size, Energy and Centrality Dependence of Pseudorapidity Distributions of Charged Particles in Relativistic Heavy Ion Collisions

TL;DR

This study measures charged particle distributions in Cu+Cu collisions at various energies, revealing that collision geometry influences the shape of pseudorapidity distributions more than the number of nucleon participants.

Contribution

It provides the first measurements of pseudorapidity distributions in Cu+Cu collisions and highlights the importance of collision geometry over participant number in shaping these distributions.

Findings

Distribution shape correlates with Npart/2A ratio.

Collision geometry influences distribution shape more than Npart.

Total charged particles depend on nucleon participants.

Abstract

We present the first measurements of the pseudorapidity distribution of primary charged particles in Cu+Cu collisions as a function of collision centrality and energy, \sqrtsnn = 22.4, 62.4 and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants. More detailed studies reveal that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same Npart/2A value rather than the same Npart value. In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies.