Coherent collective flow versus independent two-nucleon collisions in central high-energy heavy-ion collisions

Abstract

For central collisions in the reaction $^{20}\mathrm{Ne}$ + $^{238}\mathrm{U}$ at a laboratory bombarding energy per nucleon of 393 MeV, we calculate the cross section $\frac{{d}^{2}\ensuremath{\sigma}}{\mathrm{dE}d\ensuremath{\Omega}}$ for outgoing charged particles by means of a relativistic intranuclear cascade, conventional relativistic nuclear fluid dynamics, and relativistic nuclear two-fluid dynamics. The results of the three calculations are compared with recent high-multiplicity-selected central-collision experimental data of Stock et al., including contributions from protons, deuterons, and tritons. The intranuclear cascade reproduces most features of the experimental data except the observed sidewards peaking in the angular distributions at low outgoing energy, predicting instead angular distributions that are forward peaked at all outgoing energies, to within statistical errors. Conventional nuclear fluid dynamics fails to reproduce the experimental data in two important respects: (1) The predicted energy spectra in forward directions decrease too rapidly with increasing outgoing energy and (2) the predicted angular distributions are too narrow and peak at increasingly larger angles with increasing outgoing energy, opposite to the experimental trend. Nuclear two-fluid dynamics predicts sidewards peaking for central collisions at intermediate outgoing energy that is in approximate agreement with experimental results, although the calculated peak is somewhat sharper than the experimental peak. With qualifications arising from the possible importance of contributions from large impact parameters, heavy composite particles, Coulomb effects, and thermal folding, we conclude that in central high-energy heavy-ion collisions the target and projectile interpenetrate substantially but that some degree of coherent collective flow is involved.NUCLEAR REACTIONS $^{20}\mathrm{Ne}$ + $^{238}\mathrm{U}$, $\frac{{E}_{\mathrm{bom}}}{20}=393$ MeV. Calculated $\frac{{d}^{2}\ensuremath{\sigma}}{\mathrm{dE}d\ensuremath{\Omega}}$ for outgoing charged particles in central collisions and compared with experimental data. Relativistic intranuclear cascade, relativistic nuclear fluid dynamics, relativistic nuclear two-fluid dynamics, nuclear equation of state.