Inversion of collective matter flow in nucleus-nucleus collisions, measured with a 4π charged particle array

Abstract

The multidetector array Mur + Tonneau has been used to perform a 4π detection of charged particles and fragments emitted in reactions between 40Ar, at energies ranging from 25 to 85 MeV/u, and 27Al or 58Ni target. The events have been sorted as a function of the impact parameter value. The collective transverse momentum in the reaction plane (sidewards flow parameter) is observed to strongly vary as a function of impact parameter and incident energy. At low energies, it is shown to be negative. It inverts to positive values at beam energies in the range 70–100 MeV/u depending on the impact parameter value and the mass of the system. These data are compared to dynamical calculations based on LVUU and QMD. In this energy range, the calculated flow values are much more sensitive to the nucleon-nucleon cross section in medium and to the equation of state than at relativistic energies; therefore, quantitative information can be obtained.