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

Abstract

We present directed flow of protons and pions, as well as mean polarization of Λ and ¯Λ hyperons computed for Au-Au collisions at √SNN = 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 Λ polarization shows a significant splitting between polarizations of Λ and ¯Λ, which challenges a widespread interpretation that the splitting is affected mainly by the late-stage magnetic field.