Observation of $\Lambda$ hyperon local polarization in pPb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV

Abstract

The polarization of the $\Lambda$ and $\overline{\Lambda }$ hyperons along the beam direction has been measured in proton-lead (pPb) 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 $\pm$ 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 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.

arXiv commentReplaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-24-002 (CMS Public Pages)

References in corpus (60)

• Collective flow and viscosity in relativistic heavy-ion collisions
• Methods for analyzing anisotropic flow in relativistic nuclear collisions
• EPOS LHC : test of collective hadronization with LHC data
• Global $\Lambda$ hyperon polarization in nuclear collisions: evidence for the most vortical fluid
• Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC
• The CMS trigger system
• Description and performance of track and primary-vertex reconstruction with the CMS tracker
• Long-range angular correlations on the near and away side in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV
• Observation of long-range near-side angular correlations in proton-lead collisions at the LHC
• Hydrodynamic Modeling of Heavy-Ion Collisions
• Observation of Associated Near-side and Away-side Long-range Correlations in sqrt(s_NN)=5.02 TeV Proton-lead Collisions with the ATLAS Detector
• Globally Polarized Quark-gluon Plasma in Non-central A+A Collisions
• Multiplicity and transverse-momentum dependence of two- and four-particle correlations in pPb and PbPb collisions
• Evidence for collectivity in pp collisions at the LHC
• Angular momentum conservation in heavy ion collisions at very high energy
• Relativistic distribution function for particles with spin at local thermodynamical equilibrium
• Properties of hot and dense matter from relativistic heavy ion collisions
• Observation of long-range elliptic anisotropies in $\sqrt{s}=$13 and 2.76 TeV $pp$ collisions with the ATLAS detector
• Performance of the CMS Level-1 trigger in proton-proton collisions at $\sqrt{s}=$ 13 TeV
• Creating small circular, elliptical, and triangular droplets of quark-gluon plasma
• Small System Collectivity in Relativistic Hadron and Nuclear Collisions
• Chiral anomaly and local polarization effect from quantum kinetic approach
• Novel Collective Phenomena in High-Energy Proton-Proton and Proton-Nucleus Collisions
• Polarization of $\Lambda$ ($\bar{\Lambda }$) hyperons along the beam direction in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV
• Measurement of long-range near-side two-particle angular correlations in pp collisions at sqrt(s) = 13 TeV
• Collective longitudinal polarization in relativistic heavy-ion collisions at very high energy
• Polarization and Entanglement in Baryon-Antibaryon Pair Production in Electron-Positron Annihilation
• Shear-induced spin polarization in heavy-ion collisions
• Local polarization and isothermal local equilibrium in relativistic heavy ion collisions
• Probing vorticity structure in heavy-ion collisions by local $\Lambda$ polarization
• Spin-thermal shear coupling in a relativistic fluid
• Measurement of long-range angular correlation and quadrupole anisotropy of pions and (anti)protons in central $d$$+$Au collisions at $\sqrt{s_{{}_{NN}}}$=200 GeV
• Pseudorapidity distribution and decorrelation of anisotropic flow within CLVisc hydrodynamics
• Measurements of elliptic and triangular flow in high-multiplicity ${}^3$He$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Proton and Pion Production Relative to the Reaction Plane in Au + Au Collisions at AGS Energies
• Global polarization of $\Lambda$ and $\overline{\Lambda }$ hyperons in Pb-Pb collisions at the LHC
• Estimating initial state and quark-gluon plasma medium properties using a hybrid model with nucleon substructure calibrated to $p$-Pb and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV
• Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies
• Constraints on the chiral magnetic effect using charge-dependent azimuthal correlations in pPb and PbPb collisions at the LHC
• Long-range pseudorapidity dihadron correlations in $d$+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV
• Elliptic flow of charm and strange hadrons in high-multiplicity pPb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=$ 8.16 TeV
• Measurements of long-range near-side angular correlations in $\sqrt{s_{\text{NN}}}=5$TeV proton-lead collisions in the forward region
• Observation of correlated azimuthal anisotropy Fourier harmonics in pp and pPb collisions at the LHC
• (3+1)-D viscous hydrodynamics CLVisc at finite net baryon density: identified particle spectra, anisotropic flows and flow fluctuations across BES energies
• Multiplicity and rapidity dependence of strange hadron production in pp, pPb, and PbPb collisions at the LHC
• Observation of Transverse $\Lambda /\bar{\Lambda }$ Hyperon Polarization in $e^{+}{e}^{-}$ Annihilation at Belle
• Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Azimuthal harmonics in small and large collision systems at RHIC top energies
• Vorticity and particle polarization in heavy ion collisions (experimental perspective)
• Observation of prompt J/$\psi$ meson elliptic flow in high-multiplicity pPb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 8.16 TeV
• Studies of charm and beauty hadron long-range correlations in pp and pPb collisions at LHC energies
• First extraction of the $\Lambda$ polarising fragmentation function from Belle $e^{+}{e}^{-}$ data
• Overview of high-density QCD studies with the CMS experiment at the LHC
• Measurements of mass-dependent azimuthal anisotropy in central $p$$+$Au, $d$$+$Au, and ${}^3$He$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Hyperon polarization along the beam direction relative to the second and third harmonic event planes in isobar collisions at $\sqrt{s_{NN}}$ = 200 GeV
• Kinematic dependence of azimuthal anisotropies in $p$$+$Au, $d$$+$Au, ${}^3$He+Au at $\sqrt{s_{{}_{NN}}}$ = 200 GeV
• Transverse $\Lambda$ Polarization in $e^{+}{e}^{-}$ collisions
• The Polarising Fragmentation Function and the Lambda polarisation in e+ e- processes
• The transverse polarization of $\Lambda$ hyperons in $e^{+}{e}^{-}\to {\Lambda }^{\uparrow }hX$ processes within TMD factorization
• Strange hadron collectivity in pPb and PbPb collisions

Cited by in corpus (1)

• Global $\Lambda$ hyperon polarization in low-energy heavy ion collisions -- a scenario without vorticity