Speaker
Description
Recently, CMS collaboration has measured the spin polarization of $\Lambda$ hyperons along the beam direction in p+Pb collision at $\sqrt{s_{NN}}=8.16$. The second Fourier sine coefficient of the spin polarization of $\Lambda$ hyperons along the beam direction decreases as multiplicity increases. This contrasts with the trend of v2 in the p+Pb collision. This finding challenges the picture that spin polarization along the beam direction is induced by anisotropic flow.
We have implemented the 3+1 dimensional CLVisc hydrodynamics model with TRENTo-3D initial conditions to investigate the spin polarization of $\Lambda$ hyperons along the beam direction in p+Pb collisions. We have computed the second Fourier sine coefficients of spin polarization along the beam direction as functions of multiplicity, transverse momentum and pseudo-rapidity. We find that the spin polarization induced by thermal vorticity always provides an opposite contribution compared to the shear-induced polarization in p+Pb collisions. The total spin polarization computed by the current hydrodynamic model disagrees with the data measured by LHC-CMS experiments. Our findings imply that other new effects may play a crucial role in p+Pb collisions.
