Global hyperon polarization at local thermodynamic equilibrium with vorticity, magnetic field, and feed-down

The system created in ultrarelativistic nuclear collisions is known to behave as an almost ideal liquid. In noncentral collisions, because of the large orbital momentum, such a system might be the fluid with the highest vorticity ever created under laboratory conditions. Particles emerging from such a highly vorticous fluid are expected to be globally polarized with their spins on average pointing along the system angular momentum. Vorticity-induced polarization is the same for particles and antiparticles, but the intense magnetic field generated in these collisions may lead to the splitting in polarization. In this paper we outline the thermal approach to the calculation of the global polarization phenomenon for particles with spin and we discuss the details of the experimental study of this phenomenon, estimating the effect of feed-down. A general formula is derived for the polarization transfer in two-body decays and, particularly, for strong and electromagnetic decays. We find that accounting for such effects is crucial when extracting vorticity and magnetic field from the experimental data.

Figure 1

Schematic view of the collision. Arrows indicate the flow velocity field. The $+\widehat{y}$ direction is out of the page; both the orbital angular momentum and the magnetic field point into the page.