Λ Polarization and Vortex Rings in Heavy-Ion Collisions at NICA Energies

We review recent studies of vortical motion and the resulting polarization of Lambda hyperons in heavy-ion collisions at NICA energies, in particular, within the model of three-fluid dynamics (3FD). This includes predictions of the global Lambda polarization and ring structures that appear in Au+Au collisions. The global Lambda polarization in Au+Au collisions is calculated, including its rapidity and centrality dependence. The contributions of the thermal vorticity and meson-field term (proposed by Csernai, Kapusta, and Welle) to the global polarization are considered. The results are compared with data from recent STAR and HADES experiments. It is predicted that the polarization maximum is reached at root s(NN) approximate to 3 GeV if the measurements are performed with the same acceptance. It is demonstrated that a pair of vortex rings are formed, one at forward rapidities and another at backward rapidities, in ultra-central Au+Au collisions at root s(NN) > 4 GeV. The vortex rings carry information about the early stage of the collision, in particular, the stopping of baryons. It is shown that these rings can be detected by measuring the ring observable R-Lambda, even in the midrapidity region at root s(NN)= 5-20 GeV. At forward/backward rapidities, the R-Lambda signal is expected to be stronger. The possibility of observing the vortex-ring signal against the background of non-collective transverse polarization is discussed.