Orateur
Description
Relativistic heavy-ion collisions provide a unique oppotunity to study spin polarization of fermions. In the past decade, a lot of progress has been made regarding to the spin polarization of $\Lambda$ hyperon, both at experimental and theoretical level. Polarizations induced by first order gradient quantities, such as the thermal vorticity tensor, have been widely discussed and successfully explained the $\Lambda$'s global polarization. However, the $\Lambda$'s polarization along the beam direction still remains a puzzle, which is known as the "spin sign puzzle" in heavy-ion collisions. In this work, we focus on fermions at local equilibrium in a relativistic fluid. We derive, for the first time, the spin polarization induced by second order derivatives of the four-temperature vector. As a consequence, gradients of the thermal vorticity and the thermal shear tensor also have sizable contributions to the polarization, which may provide a solution of the spin sign puzzle.
