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


Hayrapetyan A., Tumasyan A., Adam W., Andrejkovic J., Benato L., Bergauer T., ...More

Physical review letters, vol.135, no.13, pp.132301, 2025 (SCI-Expanded, Scopus) identifier identifier

  • Publication Type: Article / Article
  • Volume: 135 Issue: 13
  • Publication Date: 2025
  • Doi Number: 10.1103/6ywq-gm61
  • Journal Name: Physical review letters
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, Chemical Abstracts Core, Compendex, INSPEC, zbMATH, DIALNET, Nature Index
  • Page Numbers: pp.132301
  • Middle East Technical University Affiliated: Yes

Abstract

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.