Vortex rings in heavy-ion collisions at energies $\sqrt{s_{NN}}=$ 3--30 GeV and possibility of their observation

The ring structures that appear in Au+Au collisions at collision energies $\sqrt{s_{NN}}=$ 3 – 30 GeV are studied. The calculations are performed within the model of three-fluid dynamics. It is demonstrated that a pare of vortex rings are formed, one at forward and another at backward rapidities, in ultra-central Au+Au collisions at $\sqrt{s_{NN}}>$ 4 GeV. The vortex rings carry information about early stage of the collision, in particular about the stopping of baryons. It is shown that these rings can be detected by measuring the ring observable $R_{\mathrm{\Lambda }}$ even in rapidity range $0<y<0.5$ (or $-0.5<y<0$) on the level of 0.5–1.5% at $\sqrt{s_{NN}}=$ 5 – 20 GeV. At forward/backward rapidities, the $R_{\mathrm{\Lambda }}$ signal is expected to be stronger. Possibility of observation of the vortex-ring signal against background of non-collective transverse polarization is discussed.