Directed flow and hyperon polarization at RHIC BES from multi-fluid dynamics

We present directed flow of protons and pions, as well as mean polarization of $\mathrm{\Lambda }$ and $\bar{\mathrm{\Lambda }}$ hyperons computed for Au-Au collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=\mathrm{5...19.6}$ GeV in MUFFIN model. MUlti Fluid simulation for Fast IoN collisions, or MUFFIN, is a state-of-the-art 3-fluid dynamic model for simulating heavy-ion collisions in the region from a few to a hundred GeV center-of-mass energy. Whereas MUFFIN succeeds to reproduce basic observables in the collision energy range of interest, the slope of the directed flow at mid-rapidity is much steeper as compared to the data, it has unclear EoS dependence and final-state hadronic cascade affects this observable significantly. The excitation function of the $\mathrm{\Lambda }$ polarization shows a significant splitting between polarizations of $\mathrm{\Lambda }$ and $\bar{\mathrm{\Lambda }}$, which challenges a widespread interpretation that the splitting is affected mainly by the late-stage magnetic field.