Stopping and radial flow in central58Ni+58Nicollisions between1Aand2AGeV

The production of charged pions, protons, and deuterons has been studied in central collisions of ${}^{58}\mathrm{Ni}$ on ${}^{58}\mathrm{Ni}$ at incident beam energies of $1.06A$, $1.45A$, and $1.93A$ GeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for between 10A and 200A GeV at the Brookhaven AGS and the CERN SPS accelerators energies is valid down to $1A$ GeV. The degree of nuclear stopping is discussed; quantum molecular dynamics calculations reproduce the measured proton rapidity spectra for the most central events reasonably well, but do not show any sensitivity between the soft and the hard equation of state. A radial flow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures $T$ and radial flow velocities ${\beta }_r$ which increase with beam energy up to $2A$ GeV; in comparison to the existing data of Au on Au over a large range of energies, only ${\beta }_r$ shows a system size dependence.