Measuring spin correlation between quarks during QCD confinement

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields<sup>1</sup> and a condensate of virtual quark-antiquark pairs. The spontaneous breaking of the approximate chiral symmetry<sup>2</sup>, signalled by the nonvanishing quark condensate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mi>q</mml:mi> <mml:mover><mml:mrow><mml:mi>q</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> <mml:mo>⟩</mml:mo></mml:mrow> </mml:math> , is dynamically generated through topologically nontrivial gauge configurations such as instantons<sup>3</sup>. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement<sup>4</sup> remains a profound open question in quantum chromodynamics (QCD)-the fundamental theory of strong interaction. High-energy proton-proton collisions could liberate virtual quark-antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Λ</mml:mi> <mml:mover><mml:mrow><mml:mi>Λ</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> </mml:math> hyperon pairs inherited from spin-correlated strange quark-antiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 ± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.

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Publication Details

Journal
Nature
Published
2026-02-04
DOI
https://doi.org/10.1038/s41586-025-09920-0
Citations
3
Primary Topic
Quantum Chromodynamics and Particle Interactions
Type
article
Field-Weighted Citation Impact
83.88

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article

Measuring spin correlation between quarks during QCD confinement

3 citations
Nature
Quantum Chromodynamics and Particle Interactions
83.88
article

Measuring spin correlation between quarks during QCD confinement

article en
3 citations

Abstract

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields<sup>1</sup> and a condensate of virtual quark-antiquark pairs. The spontaneous breaking of the approximate chiral symmetry<sup>2</sup>, signalled by the nonvanishing quark condensate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mi>q</mml:mi> <mml:mover><mml:mrow><mml:mi>q</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> <mml:mo>⟩</mml:mo></mml:mrow> </mml:math> , is dynamically generated through topologically nontrivial gauge configurations such as instantons<sup>3</sup>. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement<sup>4</sup> remains a profound open question in quantum chromodynamics (QCD)-the fundamental theory of strong interaction. High-energy proton-proton collisions could liberate virtual quark-antiquark pairs from the vacuum that subsequently undergo confinement to form hadrons, whose properties could serve as probes into QCD confinement and the quark condensate. Here we report evidence of spin correlations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Λ</mml:mi> <mml:mover><mml:mrow><mml:mi>Λ</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> </mml:math> hyperon pairs inherited from spin-correlated strange quark-antiquark virtual pairs. Measurements by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveal a relative polarization signal of (18 ± 4)% that links the virtual spin-correlated quark pairs from the QCD vacuum to their final-state hadron counterparts. Crucially, this correlation vanishes when the hyperon pairs are widely separated in angle, consistent with the decoherence of the quantum system. Our findings provide a new experimental model for exploring the dynamics and interplay of quark confinement and entanglement.

NatureVol. 650(8100)
Rutgers, The State University of New Jersey (US), University of California, Riverside (US), Eötvös Loránd University (HU), Warsaw University of Technology (PL), Lovely Professional University (IN), Creighton University (US), University of Science and Technology of China (CN), Variable Energy Cyclotron Centre (IN), Argonne National Laboratory (US), Indian Institute of Technology Patna (IN), State University of New York (US), University of Kentucky (US), University of Jammu (IN), University of Tsukuba (JP), Lawrence Berkeley National Laboratory (US), Valparaiso University (US), Kent State University (US), Shandong University (CN), National Institute of Technology Durgapur (IN), Chongqing University (CN), University of California, Los Angeles (US), National Institute of Science Education and Research (IN), Wayne State University (US), University of Tarapacá (CL), Lehigh University (US), South China Normal University (CN), United States Naval Academy (US), Ball State University (US), Chinese Academy of Sciences (CN), Homi Bhabha National Institute (IN), Brookhaven National Laboratory (US), Purdue University West Lafayette (US), Heidelberg University (DE), Fudan University (CN), Frankfurt Institute for Advanced Studies (DE), Sejong University (KR), Guangxi Normal University (CN), Huzhou Normal University (CN), Technische Universität Darmstadt (DE), Yale University (US), Southern Connecticut State University (US), University of Illinois Chicago (US), Central China Normal University (CN), Max Planck Institute for Physics (DE), Indian Institute of Science Education and Research Berhampur (IN), Indiana University Bloomington (US), Shanghai Institute of Applied Physics (CN), Institute of Modern Physics (CN), Indian Institute of Science Education and Research, Tirupati (IN), Czech Academy of Sciences, Nuclear Physics Institute (CZ), University of Chinese Academy of Sciences (CN), Wuhan University of Science and Technology (CN), University of Houston (US), Czech Technical University in Prague (CZ), University of Calabria (IT), Texas Southern University (US), Panjab University (IN), The Ohio State University (US), Stony Brook University (US), Abilene Christian University (US), AGH University of Krakow (PL), Rice University (US), Lanzhou University (CN), American University in Cairo (EG), University of California, Davis (US), Temple University (US), Academia Sinica (TW), The University of Texas at Austin (US), Michigan State University (US), Texas A&M University (US), National Cheng Kung University (TW), University of California, Berkeley (US), Tsinghua University (CN)
National Science Foundation, U.S. Department of Energy, National Energy Research Scientific Computing Center, Steno Diabetes Center Copenhagen, National Research Foundation, Ministry of Education, Culture, Sports, Science and Technology, National Natural Science Foundation of China, Grantová Agentura České Republiky, Bundesministerium für Bildung und Forschung, Chinese Academy of Sciences, National Research Foundation of Korea, Emberi Eroforrások Minisztériuma, Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, National Science and Technology Council, Office of Science, Japan Society for the Promotion of Science, National Research, Development and Innovation Office, Nuclear Physics, Lawrence Berkeley National Laboratory
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Quantum Chromodynamics and Particle Interactions
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