Probing QCD Confinement with Spin Entanglement

by Jan Vanek (BNL)

Tuesday 22 Jul 2025, 11:00 → 12:00 US/Eastern

Small seminar

One of the main current puzzles in nuclear physics is the exact origin of the spin of the proton. It is now experimentally established that the valence quarks account only for about a third of the proton spin. The remaining 2/3 are then about equally split between sea quarks and gluons which originate from the QCD vacuum which contains a condensate of quark-antiquark pairs: $u\bar{u}$, $d\bar{d}$, and $s\bar{s}$. Due to the vacuum’s quantum numbers, $J^{PC} = 0^{++}$, these quark pairs are expected to appear as maximally entangled spin-triplet states with aligned spins. Until now, the quark condensate has not yet been directly experimentally confirmed, but a recent proposal suggests that the $s\bar{s}$ pairs in the quark condensate might be experimentally probed through measurement of spin-spin correlations of $\Lambda\bar{\Lambda}$ hyperon pairs. In this seminar, I will present the first experimental measurements of spin-spin correlations of $\Lambda\bar{\Lambda}$, $\Lambda\Lambda$, and $\bar{\Lambda}\bar{\Lambda}$ pairs in $p+p$ collisions at $\sqrt{s} = 200\,\textrm{GeV}$, recorded by the STAR detector in 2012. For the first time, a significant spin-spin correlation is observed in short-range  ($|\Delta y| < 0.5$ and $|\Delta \phi| < \pi/3$) $\Lambda\bar{\Lambda}$ pairs which is consistent with their origin in the hadronization of maximally spin-entangled $s\bar{s}$ pairs from the QCD vacuum. I will discuss the importance and physics implications of this groundbreaking observation which will provide valuable insight into a number of topics, such as the strange quark hadronization, the spin structure of the $\Lambda$ hyperon, and mechanisms of the spin transfer from polarized protons to $\Lambda$ hyperons.