Observation of <math display="inline"><mi mathvariant="normal">Λ</mi></math> Hyperon Local Polarization in <math display="inline"><mi>p</mi></math>-Pb Collisions at <math display="inline"><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>8.16</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>

Benecke, Anna;Bethani, Agni;Bruno, Giacomo;Caputo, Claudio;CMS;et.al.
(2025) Physical Review Letters — Vol. 135 (2025)

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Abstract
The polarization of the <math display="inline"><mi mathvariant="normal">Λ</mi></math> and <math display="inline"><mover accent="true"><mi mathvariant="normal">Λ</mi><mo stretchy="false">¯</mo></mover></math> hyperons along the beam direction has been measured in proton-lead (<math display="inline"><mi>p</mi><mtext>-Pb</mtext></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 <math display="inline"><mrow><mn>186.0</mn><mo>±</mo><mn>6.5</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>nb</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math>. A significant azimuthal dependence of the hyperon polarization, characterized by the second-order Fourier sine coefficient <math display="inline"><msub><mi>P</mi><mrow><mi>z</mi><mo>,</mo><mi mathvariant="normal">s</mi><mn>2</mn></mrow></msub></math>, is observed. The <math display="inline"><msub><mi>P</mi><mrow><mi>z</mi><mo>,</mo><mi mathvariant="normal">s</mi><mn>2</mn></mrow></msub></math> values decrease as a function of charged particle multiplicity, but increase with transverse momentum. A hydrodynamic model that describes the observed <math display="inline"><msub><mi>P</mi><mrow><mi>z</mi><mo>,</mo><mi mathvariant="normal">s</mi><mn>2</mn></mrow></msub></math> values in nucleus-nucleus collisions by introducing vorticity effects does not reproduce either the sign or the magnitude of the <math display="inline"><mi>p</mi><mtext>-Pb</mtext></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.
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Benecke, A., Bethani, A., Bruno, G., Caputo, C., de Favereau de Jeneret, J., Delaere, C., Donertas, I. S., Giammanco, A., Guzel, A. O., Jain, S., Jain, S., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Tran, T. T., CMS, & et al. (2025). Observation of <math display=“inline”><mi mathvariant=“normal”>Λ</mi></math> Hyperon Local Polarization in <math display=“inline”><mi>p</mi></math>-Pb Collisions at <math display=“inline”><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>8.16</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>. Physical Review Letters, 135. https://doi.org/10.1103/6ywq-gm61 (Original work published 2025)