Seminar in Hadronic Physics

Global Hyperon Polarization in Ultra-relativistic Heavy Ion Collisions Measured by the STAR Experiment at RHIC

Mike Lisa

Ohio State University

Non-central collisions between ultra-relativistic heavy ions involve millions of h-bar of angular momentum. It is unclear how much- if any- of this angular momentum is transferred to the quark-gluon plasma created at midrapidity. Such a transfer may arise via a spin-orbit coupling in QCD or, in a hydrodynamic picture, through shear forces that generate a vorticity in the fluid. While thermalization of energy and light-flavor chemical degrees of freedom in heavy ion collisions is well-established, thermal distribution of angular momentum among all available substates is far from clear. Angular momentum thermalization or vorticity in the QGP may result in a correlation between the spin of emitted hadrons and the direction of the angular momentum of the overall system. This “global polarization” is distinct from polarization relative to the particle production plane observed at very forward angles in p+p collisions. Due to their so-called self-analyzing nature, hyperons reveal their spin orientation in their decay. STAR has measured the first non-zero signal of the polarization of Lambdas and AntiLambdas relative to the direction of the collision's angular momentum, estimated by the event reaction plane, at several collision energies. I will discuss details of the analysis, the energy dependence of the signal, and two methods of quantifying the polarization. I will also discuss detector and accelerator upgrades that will allow us to study this new signal in far greater detail in the upcoming second phase of the Beam Energy Scan at RHIC.