Experiments at the Relativistic Heavy Ion Collider have measured the net polarization of Λ and Λ hyperons and attributed it to a coupling between their spin and the vorticity of the fluid created in heavy ion collisions, but how the spin comes to equilibrium with vorticity is an open problem. Recently, we found that vorticity fluctuations and helicity flip of strange quarks in quark-gluon plasma through perturbative QCD processes resulted in equilibration times far too long to be relevant. Here, we consider the Nambu-Jona-Lasinio model with the inclusion of the six-quark Kobayashi-Maskawa-'t Hooft interaction which breaks axial U(1). Using instanton inspired models for the temperature dependence of the axial symmetry breaking, we find that constituent strange quarks can reach spin equilibrium at temperatures below about 170 MeV, just before they hadronize to form hyperons.
Bibliographical noteFunding Information:
The work of J.I.K. was supported by the US Department of Energy Grant No. DE-FG02-87ER40328. The work of E.R. was supported by the US National Science Foundation Grant No. PHY-1630782 and by the Heising-Simons Foundation Grant No. 2017-228.
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