Polarized Baryon Production in Heavy Ion Collisions: An Analytic Hydrodynamical Study

Abstract

We utilize known exact analytic solutions of perfect fluid hydrodynamics to analytically calculate the polarization of baryons produced in heavy ion collisions. Assuming local thermodynamical equilibrium also for spin degrees of freedom, baryons get a net polarization at their formation (freeze-out). This polarization depends on the time evolution of the Quark-Gluon Plasma (QGP), which can be described as an almost perfect fluid. By using exact analytic solutions, we thus can analyze the necessity of rotation (and vorticity) for non-zero net polarization. In this paper we give the first analytical calculations for the polarization four-vector. We use two hydrodynamical solutions; one is the spherically symmetric Hubble flow (a somewhat oversimplified model, to demonstrate the methodology). The other solution which we use is a somewhat more involved one that corresponds to a rotating and accelerating expansion, and is thus well suited to investigate some main features of the time evolution of the QGP created in peripheral heavy-ion collisions (although there are still many numerous features of a real collision geometry that are beyond the reach of this simple model). Finally we illustrate and discuss our results on the polarization.

Longitudinal spin polarization in a thermal model

“…For our further considerations, it is useful to notice that most of the theoretical frameworks used to describe spin polarization deal with particles at freeze-out [13,14]. This is natural in the approaches that directly connect spin polarization with the "vortical" properties of the fluid [7,15,16]. In such a scenario, the spin polarization tensor ω µν follows immediately the space-time changes of the thermal vorticity µν , hence, it is enough to consider the two quantities at freeze-out.…”

Analytic model studies of polarized baryon production

“…In the work presented here [7] we set out to give analytic formulas for the polarization of massive spin 1/2 particles, based on the simple formula written up in Ref. [2] under the assumption of local thermal equilibrium.…”