Lambda polarization in heavy ion collisions: from RHIC BES to LHC energies

Abstract

STAR collaboration at RHIC has recently measured the polarization of Λ hyperons in non-central heavy ion collisions in the RHIC Beam Energy Scan (BES) program. The magnitude of the polarization was found to decrease from few percents at the lowest BES energies to ≈ 0.2% at the top RHIC energy. The polarization signal has been reproduced in different hydrodynamic calculations assuming a thermodynamic spin-vorticity coupling mechanism at the Cooper-Frye hypersurface [1].In this work an extension of our existing calculations [2] of the Λ polarization in the RHIC BES program to the top RHIC and 2.76 TeV LHC energies is presented. The longitudinal component of the Λ polarization, which is the dominant component of the polarization at the LHC energies, is discussed. Finally we show that the global polarization of Λ originates dominantly from the relativistic analogue of the classical vorticity, whereas the quadrupole longitudinal component originates from the gradients of temperature and acceleration of the medium when the Λs are produced out of the fluid.