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

Abstract

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 $p\ensuremath{\alpha}$, $\mathrm{dd}$, $\ensuremath{\alpha}\ensuremath{\alpha}$, $\mathrm{pA}$, ${\ensuremath{\pi}}^{+}A$, ${K}^{+}A$, 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.