Droplet formation in expanding nuclear matter: a system-size dependent study

Cluster production is investigated in central collisions of Ca+Ca, Ni+Ni, ⁹⁶Zr+⁹⁶Zr, ⁹⁶Ru+⁹⁶Ru, Xe+CsI and Au+Au reactions at 0.4 AGeV incident energy. We find that the multiplicity of clusters with charge Z >= 3 grows quadratically with the system's total charge and is associated with a mid-rapidity source with increasing transverse velocity fluctuations. When reduced to the same number of available charges, an increase of cluster production by about a factor of 5.5 is observed in the mid-rapidity region between the lightest system (Ca+Ca) and the heaviest one (Au+Au). The results, as well as simulations using Quantum Molecular Dynamics, suggest a collision process where droplets, i.e. nucleon clusters, are created in an expanding, gradually cooling, nucleon gas. Within this picture, expansion dynamics, collective radial flow and cluster formation are closely linked as a result of the combined action of nucleon-nucleon scatterings and the mean fields.