Observation of Λ Hyperon Local Polarization in p -Pb Collisions at sNN</…

The polarization of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi mathvariant="normal">Λ</a:mi></a:math> and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mover accent="true"><d:mi mathvariant="normal">Λ</d:mi><d:mo stretchy="false">¯</d:mo></d:mover></d:math> hyperons along the beam direction has been measured in proton-lead (<i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>p</i:mi><i:mtext>-Pb</i:mtext></i:math>) 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 <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mn>186.0</k:mn><k:mo>±</k:mo><k:mn>6.5</k:mn><k:mtext> </k:mtext><k:mtext> </k:mtext><k:msup><k:mrow><k:mi>nb</k:mi></k:mrow><k:mrow><k:mo>−</k:mo><k:mn>1</k:mn></k:mrow></k:msup></k:mrow></k:math>. A significant azimuthal dependence of the hyperon polarization, characterized by the second-order Fourier sine coefficient <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>P</m:mi><m:mrow><m:mi>z</m:mi><m:mo>,</m:mo><m:mi mathvariant="normal">s</m:mi><m:mn>2</m:mn></m:mrow></m:msub></m:math>, is observed. The <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:msub><p:mi>P</p:mi><p:mrow><p:mi>z</p:mi><p:mo>,</p:mo><p:mi mathvariant="normal">s</p:mi><p:mn>2</p:mn></p:mrow></p:msub></p:math> values decrease as a function of charged particle multiplicity, but increase with transverse momentum. A hydrodynamic model that describes the observed <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:msub><s:mi>P</s:mi><s:mrow><s:mi>z</s:mi><s:mo>,</s:mo><s:mi mathvariant="normal">s</s:mi><s:mn>2</s:mn></s:mrow></s:msub></s:math> values in nucleus-nucleus collisions by introducing vorticity effects does not reproduce either the sign or the magnitude of the <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mi>p</v:mi><v:mtext>-Pb</v:mtext></v:math> 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.