Lambda polarization in $^{108}$Ag +$^{108}$Ag and $^{197}$Au +$^{197}$Au collisions around a few GeV

Abstract

Within the framework of Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we extract the global spin polarization of Λ hyperon in 108 Ag + 108 Ag and 197 Au + 197 Au collisions at √ sNN = 2.42 − 7.7 GeV. We use two different methods to calculate the Λ polarization Py: one is based on equilibrium assumption so that Py is determined by thermal vorticity and another assumes a proportional relation between Py and the system's angular momentum in Λ's rest frame. We find that both methods can describe the experimental data suggesting that the global spin polarization is a result of global angular momentum and is thus insensitive to the intermediate prescription to extracting it.

Spin Polarization Formula for Dirac Fermions at Local Equilibrium

“…where s = 1/2 is the spin quantum number. Based on this formula, the experimental data for global spin polarization of hyperons can be well reproduced from theoretical computations [12][13][14][15][16][17][18][19][20][21][22][23][24]. However, when applying the same formula to the calculation of local spin polarization (i.e., the differential spin polarization as a function of the azimuthal angle), the results deviate substantially from the data [25][26][27][28][29][30][31][32][33].…”

Spin and polarization: a new direction in relativistic heavy ion physics

“…The experiment STAR has been able to push the lowest energy polarization measurement down to √ s NN = 3 GeV in order to test the typical prediction of the hydrodynamic model of an increasing polarization as energy decreases [13]. It is not yet clear if the same hydrodynamic model which is used at high energy can be applied at such a low energy, however there are calculations [33,34,35,36] based on hydrodynamics and other models predicting the appearance of a peak of global polarization as a function of collision energy. The magnitude measured by the collaboration STAR [7] seems to favour the three-fluid model [33], however it should be stressed that none of the above calculations included the spin-shear term (8).…”

Spin Effects In Heavy Ion Collisions at High Energies

“…The HADES results [29] at such low energy seem to suggest that GPE continue to rise with decreasing energy. This is a place worthwhile for further studies and theoretical efforts have already been made and/or are underway [48][49][50][51][52].…”