Charged-particle multiplicity and pseudorapidity distributions measured with the PHOBOS detector in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mtext>Au</mml:mtext><mml:mo>+</mml:mo><mml:mtext>Au</mml:mtext></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mtext>Cu</mml:mtext><mml:mo>+</mml:mo><mml:mtext>Cu</mml:mtext></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="bold">d</mml:mi><mml:mo fontweight="bold" fontstyle="normal">+</mml:mo><mml:mtext>Au</mml:mtext></mml:mrow></mml:math>, and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="bold">p</mml:mi><mml:mo fontweight="bold" fontstyle="normal">+</mml:mo><mml:mi mathvariant="bold">p</mml:mi></mml:mrow></mml:math>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 $|\ensuremath{\eta}|&lt;5.4$, 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.