Is nucleon spin thermalized in intermediate-energy heavy-ion collisions?

Abstract

Despite the success of the spin-thermalized assumption in explaining hyperon spin polarizations in relativistic heavy-ion collisions, challenges begin to arise especially at lower collision energies. The present study compares the nucleon spin polarization during the collision process and at the freeze-out stage from a non-relativistic spin-dependent transport model with spin-thermalized approaches in intermediate-energy heavy-ion collisions, where the relativistic effect and the temperature gradient have shown to be unimportant. It is found that both the global and local spin polarizations are largely overestimated from spin-thermalized approaches, compared to those generated by the spin-orbit mean-field potential in transport simulations.

Figures (6)

• Figure 1: Reduced density $\rho/\rho_0$ (first row), temperature $T$ (second row), and the $y$-component of the non-relativistic (third row) and covariant (fourth row) kinematic vorticity in the reaction ($x-o-z$) plane at different times in non-central Au+Au collisions at the beam energy of 100 AMeV.
• Figure 2: $y$-component of the spin density $s_y$ (first row) and the corresponding nucleon spin polarization $P_y=s_y/\rho$ (second row) in $y$ direction from SIBUU, as well as the nucleon polarization $P_y$ from the kinematic vorticity (third row), the thermal vorticity (fourth row), and the spin vector (fifth row) in the reaction plane at different times in non-central Au+Au collisions at the beam energy of 100 AMeV.
• Figure 3: Rapidity dependence of the spin polarization $P_y$ for free nucleons in non-central Au+Au collisions at the beam energy of 100 (a), 50 (b), and 150 (c) AMeV, from SIBUU with different spin-orbit coupling coefficients $W_0$, and various approaches based on the spin-thermalized assumption.
• Figure 4: Reduced density $\rho/\rho_0$ (first row), temperature $T$ (second row), and $z$-component of the non-relativistic (third row) and covariant (fourth row) kinematic vorticity in the transverse ($x-o-y$) plane at different times in non-central Au+Au collisions at the beam energy of 100 AMeV.
• Figure 5: $z$-component of the spin density $s_z$ (first row) and the corresponding nucleon spin polarization $P_z=s_z/\rho$ (second row) in $z$ direction from SIBUU, as well as the nucleon polarization $P_z$ from the kinematic vorticity (third row), the thermal vorticity (fourth row), and the spin vector (fifth row) in the reaction plane at different times in non-central Au+Au collisions at the beam energy of 100 AMeV.