Charged-particle multiplicity and pseudorapidity distributions measured with the PHOBOS detector in Au+Au , Cu+Cu , d+Au , and p+p collisions at ultrarelativistic energies

Pseudorapidity distributions of charged particles emitted in $\text{Au}+\text{Au}$, $\text{Cu}+\text{Cu}$, $d+\text{Au}$, and $p+p$ collisions over a wide energy range have been measured using the PHOBOS detector at the BNL Relativistic Heavy-Ion Collider (RHIC). The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with $|\ensuremath{\eta}|<5.4$, 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 ${\mathit{dN}}_{\text{ch}}/d\ensuremath{\eta}$ and the total charged-particle multiplicity ${N}_{\text{ch}}$ are found to factorize into a product of independent functions of collision energy, $\sqrt{{s}_{{}_{\mathit{NN}}}}$, and centrality given in terms of the number of nucleons participating in the collision, ${N}_{\text{part}}$. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of $(\mathrm{ln}{s}_{{}_{\mathit{NN}}}){}^{2}$ over the full range of collision energy of $\sqrt{{s}_{{}_{\mathit{NN}}}}=2.7$--200 GeV.