Observation of Lambda hyperon local polarization in pPb collisions at sqrt(s_NN) = 8.16 TeV

The polarization of the Λ and ¯Λ 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 𝑃𝑧,s⁢2, is observed. The 𝑃𝑧,s⁢2 values decrease as a function of charged particle multiplicity, but increase with transverse momentum. A hydrodynamic model that describes the observed 𝑃𝑧,s⁢2 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.