Azimuthal dependence of the directed flow in collisions up to 95 MeV/u: From in-plane to out-of-plane enhancement

Nucleus-nucleus reaction mechanisms at incident energies below 10 MeV/u are governed by mean field effects. Nucleon-nucleon collisions begin to become important as the energy increases above the Fermi energy. This evolution manifests itself experimentally in the modification of the directed flow of the emitted participant (mid-rapidity) nucleons. In the reaction plane, it is expressed by the flow parameter [1]. Another aspect lies in the azimuthal distribution of particles relative to the reaction plane. At high energies, a maximum in the direction perpendicular to the reaction plane on both sides was named squeeze-out-effect [2]. Conversely, below 100 MeV/u, maxima in the reaction plane have been observed for the light systems of Ar + V: rotation-like effect [3]. It was studied as a function of energy, impact parameter and emitted particle mass in the collisions of ⁴⁰Ar on ²⁷Al at energies from 36 to 65 MeV/u [4]. The latter study has been extended up to 95 MeV/u and measurements have been made for the ⁶⁴Zn + ⁵⁸Ni system at 5 energies from 35 to 79 MeV/u.