Measuring spin correlation between quarks during QCD confinement

The vacuum is now understood to have a rich and complex structure, characterized by fluctuating energy fields¹ and a condensate of virtual quark-antiquark pairs. The spontaneous breaking of the approximate chiral symmetry², 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 instantons³. The precise mechanism linking the chiral symmetry breaking to the mass generation associated with quark confinement⁴ 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.