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

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 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 vertical bar eta vertical bar < 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, root 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 root S-NN = 2.7-200 GeV.