Vorticity and $\mathrm{\Lambda }$ polarization in the microscopic transport model PACIAE

We study the behavior of four types of vorticities in noncentral $\text{Au}+\text{Au}$ collisions at energies $\sqrt{S_{NN}}=5\text{–}200$ GeV using a microscopic transport model PACIAE. The results show that the vorticities decay faster at lower energy and smaller impact parameter. The nonmonotonic dependence of the initial vorticities on the collision energies is reconfirmed and the turning point is 10–15 GeV for different vorticities. The global $\mathrm{\Lambda }$ polarization at energies $\sqrt{S_{NN}}=7.7\text{–}200$ GeV is reproduced by introducing an energy density freeze-out criterion.

Figure 1

Schematic picture of the coordinate system in the PACIAE model. Panel (a) is in the transverse plane and panel (b) is in the reaction plane.

Figure 2

Schematic picture of the generalized coarse-graining.

Figure 3

Time evolution of the four types of average vorticity at different energies in the PACIAE model.

Figure 4

Time evolution of the four types of average vorticity at different impact parameters in the PACIAE model.

Figure 5

The spatial distributions of four types of vorticities in the reaction plane with $b=7$ at $t=4$ fm/$c$ for 11.5 GeV in the PACIAE model.

Figure 6

Four types of initial vorticities at $t=0.5$ fm/$c$ (blue solid square) and $t=1$ fm/$c$ (red solid circle) as a function of the collision energy at $b=7$ fm.

Figure 7

The global $\mathrm{\Lambda }$ polarization of $\text{Au}+\text{Au}$ collision as a function of collision energy with $b=7$ fm in PACIAE compared with STAR experimental data [26] (red open star and blue open inverted triangle). The results of this work are shown by the green solid circles.