Initial energy density of quark-gluon plasma in relativistic heavy-ion collisions

We estimate the initial rapidity distribution and the initial energy density in the central rapidity region of relativistic heavy-ion collisions by using a multiple-collision model and the nuclear-thickness function of Glauber. The parameter of the rapidity distribution is determined from the experimental multiplicity data of $p\alpha$, $\mathrm{dd}$, $\alpha \alpha$, $\mathrm{pA}$, ${\pi }^{+}A$, $K^{+}A$, Si+Ag, and Ca+C reactions. We find that the initial energy density in the central rapidity region is high. For example, for the head-on collision of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ on ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ at 30 GeV per nucleon in the center-of-mass system, the maximum energy density is about 10 GeV/${\mathrm{fm}}^3$, which may exceed the critical energy density for a phase transition from a confined hadron matter to an unconfined quark-gluon plasma. The initial energy density goes as $A^{\frac{1}{3}}{B}^{\frac{1}{3}}$ for the collision of two nuclei with mass numbers $A$ and $B$, and is rather insensitive to impact parameters.