Origin of the large global spin alignment of $\phi$ mesons
In plain words
In off-center heavy-ion collisions the produced matter rotates, and spinning particles should line up slightly with the rotation. STAR found that $\phi$ mesons (strange quark plus strange antiquark) line up far more than expected, while K*0 mesons do not.
Precise statement
STAR measured the spin density-matrix element $\rho_{00}$ of $\phi$ and $K*0$ relative to the reaction plane in $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC (Nature, 2023, doi 10.1038/s41586-022-05557-5): $\rho_{00}(\phi)$ exceeds $1/3$ well beyond the size expected from vorticity and electromagnetic fields, while $\rho_{00}(K*0)$ is consistent with $1/3$. Determine the mechanism. Answer: a model reproducing $\rho_{00}$ for $\phi$ and $K*0$ versus collision energy, centrality and $p_{T}$, with predictions for $J/\psi$ and other vector mesons.
What would settle it
Measurements of $\rho_{00}$ for $\phi$, $K*0$ and $J/\psi$ versus energy, rapidity and system size, compared with models that include fluctuating strong-interaction vector fields and hadronization effects.
Status in the literature
Unverified note
The 2023 result was attributed by some authors to local fluctuations of strong-force fields, without consensus as of 2026.