Origin of the large global spin alignment of $\phi$ mesons

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.

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.