Seminar in Hadronic Physics

Polarization and helicity of lambda hyperons across RHIC-BES energies

Xiang-Yu Wu

McGill University

By measuring the polarization of lambda hyperons, the STAR collaboration at RHIC has discovered the most vortical fluid in nature. This high vorticity is a result of the huge initial orbital angular momentum carried by quark-gluon plasma (QGP) in non-central heavy-ion collisions. The rapidly rotating fluid, due to spin-orbit coupling, can induce the spin polarization of the emitted hadrons, perpendicular to the reaction plane. This phenomenon is also referred to as the global polarization of lambda hyperons. Additionally, the inhomogeneous expansion of the anisotropic QGP fireball in non-central collisions can create a periodic azimuthal angle dependence in the vorticity structure, leading to local spin polarization of lambda hyperons. Compared to traditional anisotropic collective flow observables, the spin polarization of lambda hyperons provides us with new insights into the detailed structure and transport properties of the QGP medium. In this talk, I will first introduce a new integrated event-by-event framework by extending the (3 + 1)-dimensional CLVisc viscous hydrodynamics to the region with finite net baryon number density. Then, based on this new CLVisc hydrodynamics framework with the AMPT initial model and SMASH initial conditions, I will perform a systematic study on the global and local spin polarization of lambda hyperons in Au + Au collisions across BES energies. We will discuss in detail the contributions from the spin hall effect, initial conditions, and baryon diffusion effects. In the final part of my talk, I will briefly introduce a newly defined observable associated with spin, helicity polarization.