Can the nuclear equation of state be measured in heavy-ion reactions at SIS energies?

Abstract

The quantum molecular dynamics (QMD) approach is so far the only theory which can predict the transverse momentum transfer in high-energy heavy-ion collisions quantitatively. Boltzmann-Uehling-Uhlenbeck (BUU) type calculations overpredict the transverse momentum transfer by at least 50% for heavy projectile-target combinations at energies around 800 MeV/nucleon. With an improved version of the QMD, which conserves energy better than 1%, we investigate the consequences of this small momentum transfer especially the time evolution of the high-density zone created in central reactions Au(800 MeV/nucleon) + Au. We find that on the average the potential energy is lowered and only the most central nucleons feel a compressional energy of about 15 MeV/nucleon. The “bounce-off”, i.e. the transverse momentum transfer is not due to the release of compressional energy but caused to avoid compression and hence determined by the density gradient. This explains why BUU calculations are bound to overpredict the magnitude. Thus the transverse momentum transfer is only a quite indirect measure for the nuclear equation of state. The small compressional energy raises the question of whether the present accelerators are too low in energy to determine the nuclear equation of state.