Observation of Λ Hyperon Local Polarization in p-Pb Collisions at sqrt[s_{NN}]=8.16  TeV

The polarization of the Λ and Λ[over ¯] hyperons along the beam direction has been measured in proton-lead (p-Pb) collisions at a center-of-mass energy per nucleon pair of 8.16 TeV. The data were obtained with the CMS detector at the LHC and correspond to an integrated luminosity of 186.0±6.5  nb^{-1}. A significant azimuthal dependence of the hyperon polarization, characterized by the second-order Fourier sine coefficient P_{z,s2}, is observed. The P_{z,s2} values decrease as a function of charged particle multiplicity, but increase with transverse momentum. A hydrodynamic model that describes the observed P_{z,s2} values in nucleus-nucleus collisions by introducing vorticity effects does not reproduce either the sign or the magnitude of the p-Pb 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.