Charged particle pseudo-rapidity density per participant pair for central nucleus-nucleus [17-25] and non-single diffractive pp ($\mathrm {p\overline{p}}$) collisions [26-32],as a function of $\snn$ The solid lines $\propto s_{\rm NN}^{0.15}$ and $\propto s_{\rm NN}^{0.11}$ are superimposed on the heavy-ion and pp ($\mathrm {p\overline{p}}$) data, respectively.