Speaker
Description
The observation of hyperon polarization along beam direction ($P_z$) in nucleus-nucleus collisions has opened a new way to study the complex vortical structures of the QGP. With the high-statistics data collected by the CMS experiment, we present the first $P_z$ results for $\Lambda$ and $\bar{\Lambda}$ particles in pPb collision at $\sqrt{s_{NN}}= 8.16$ TeV over a wide transverse momentum ($p_{T}$) and charged particle multiplicity ($N_\mathrm{trk}^\mathrm{offline}$) range. The $P_z$ values decrease as a function of $N_\mathrm{trk}^\mathrm{offline}$, but increase with $p_{T}$. A hydrodynamic model that describes the observed $P_z$ values in nucleus-nucleus collisions by introducing vorticity effects does not reproduce either the sign or the magnitude of the pPb results. These observations pose a challenge to the current theoretical implementation of spin polarization in heavy ion collisions and offer new insights into the origin of spin polarization in hadronic collisions at LHC energies.
