Phobos results on charged particle multiplicity and pseudorapidity distributions in Au+Au, Cu+Cu, d+Au, and p+p collisions at ultra-relativistic energies

TL;DR

This paper reports measurements of charged particle pseudorapidity distributions in various heavy-ion and proton collisions at RHIC, revealing factorization of distributions and a linear dependence of total multiplicity on the squared logarithm of collision energy.

Contribution

It provides comprehensive measurements of pseudorapidity distributions across different collision systems and energies, demonstrating factorization and a specific energy dependence of total multiplicity.

Findings

Pseudorapidity distributions account for 95-99% of charged particles.

Distributions factorize into energy and centrality functions.

Total multiplicity scales with $( ext{ln } s_{_{NN}})^2$.

Abstract

Pseudorapidity distributions of charged particles emitted in $Au+Au$, $Cu+Cu$, $d+Au$, and $p+p$ collisions over a wide energy range have been measured using the PHOBOS detector at RHIC. The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with $|\eta|<5.4$, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density, $dN_{ch}/d\eta$, and the total charged-particle multiplicity, $N_{ch}$, are found to factorize into a product of independent functions of collision energy, $\sqrt{s_{_{NN}}}$, and centrality given in terms of the number of nucleons participating in the collision, $N_{part}$. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of $(\ln s_{_{NN}})^2$ over the full range of collision energy of $\sqrt{s_{_{NN}}}$=2.7-200 GeV.