Observation of Global Hyperon Polarization in Ultrarelativistic Heavy-Ion Collisions

Isaac Upsal

doi:10.1016/j.nuclphysa.2017.05.118

Abstract

Collisions between heavy nuclei at ultra-relativistic energies form a color-deconfined state of matter known as the quark-gluon plasma. This state is well described by hydrodynamics, and non-central collisions are expected to produce a fluid characterized by strong vorticity in the presence of strong external magnetic fields. The STAR Collaboration at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) has measured collisions between gold nuclei at center of mass energies sNN=7.7–200 GeV. We report the first observation of globally polarized Λ and Λ¯ hyperons, aligned with the angular momentum of the colliding system. These measurements provide important information on partonic spin-orbit coupling, the vorticity of the quark-gluon plasma, and the magnetic field generated in the collision.

Highlights

Collisions of nuclei at ultra-relativistic energies create a system of deconfined colored quarks and gluons, called the quark-gluon plasma (QGP)
The large angular momentum (∼104−5 ) present in non-central collisions may produce a polarized QGP, in which quarks are polarized through spin-orbit coupling in QCD [1, 2, 3]
Global hyperon polarization refers to the phenomenon in which the spin of Λ hyperons is correlated with the net angular momentum of the QGP which is perpendicular to the reaction plane, spanned by pbeam and b, where b is the impact parameter vector of the collision and pbeam is the beam momentum

Summary

Introduction

Collisions of nuclei at ultra-relativistic energies create a system of deconfined colored quarks and gluons, called the quark-gluon plasma (QGP). The large angular momentum (∼104−5 ) present in non-central collisions may produce a polarized QGP, in which quarks are polarized through spin-orbit coupling in QCD [1, 2, 3]. Hyperon polarization may arise due to a coupling of the hadronic magnetic dipole moment μH to the magnetic field B produced in a non-central heavy-ion collision.

Results

Conclusion