Lambda polarization in 108Ag[formula omitted]Ag and 197Au[formula omitted]Au collisions around a few GeV.

Within the framework of Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we extract the global spin polarization of Λ hyperon in 108Ag + 108 Ag and 197 Au + 197 Au collisions at s NN = 2.42 –62.4 GeV. We use two different approaches to calculate the Λ polarization P y : approach I is based on equilibrium assumption so that P y is determined by thermal vorticity and approach II assumes a proportional relation between P y and the system's angular momentum in Λ's rest frame. We find that both approaches can describe the experimental data at low energies around a few GeV but only approach I describes well also higher-energy data. This suggests that at higher energies the relativistic effect plays an important role. After taking into such effect properly, the relativity-improved approach II can describe the higher-energy data as well. [ABSTRACT FROM AUTHOR]