On the origin of the elliptic flow and its dependence on the Equation of State in heavy-ion reactions at intermediate energies

International audienceRecently it has been discovered that the elliptic flow, v2, of composite charged particles emitted at midrapidity in heavy-ion collisions at intermediate energies shows the strongest sensitivity to the Nuclear Equation of State (EoS) which has been observed up to now within a microscopic model. This dependence on the nuclear EoS is predicted by Quantum Molecular Dynamics (QMD) calculations (Le Fevre A. ` et al., Nucl. Phys. A, 945 (2016) 112.) which show as well that the absorption or rescattering of in-plane emitted particles by the spectator matter is not the main reason for the EoS dependence of the elliptic flow at mid-rapidity but different density gradients (and therefore different forces) in the direction of the impact parameter (x-direction) as compared to the direction perpendicular to the reaction plan (y-direction), caused by the presence of the spectator matter. The stronger density gradient in the y-direction accelerates the particles more and creates therefore a negative v2. When using a soft momentum-dependent EoS, the QMD calculations reproduce the experimental results