Calculation of the expansion rate of the three-volume measure in high-energy heavy-ion collisions

In ultrarelativistic heavy-ion collisions the local three-volume measure is expanding in the longitudinal and transverse directions. This is similar t o the Hubble-expansion of the universe, except that the former is not local ly isotropic. As an example the expansion rate is calculated assuming that the energy-momentum tensor in the central region is that of an ideal fluid, undergoing Bjorken flow in longitudinal direction, and with initial condit ions as expected for BNL-RHIC energy. While the longitudinal expansion of t hree-volume is independent of the energy density of the fluid, in case of 3 + 1 dimensional expansion the form of the hydrodynamical solution (rarefac tion wave or deflagration shock) affects the three-volume expansion rate on the hadronization hypersurface. As a consequence the avel age expansion ra te on that surface ;depends on the transverse size of the system. This may reflect in an impact-parameter dependence of the formation probability of l ight nuclei and of the freeze-out temperature of the strong interactions in the system. (C) 1999 Published by Elsevier Science B.V. All rights reserve d.