What can we learn from global spin alignment of $\phi$ meson in heavy-ion collisions?

We propose that a significant positive deviation from 1/3 for the spin density matrix element $\rho_{00}$ of the $\phi$ meson may indicate the existence of a mean field of the $\phi$ meson generated in heavy-ion collisions. This explains why STAR preliminary data for the $\phi$ meson's $\rho_{00}$ are much larger than 1/3 while the data of $\Lambda$ and $\bar{\Lambda}$ polarization seem not to allow such a significant and positive deviation. The contribution may be from the polarization of the strange quark and antiquark through the $\phi$ field, an effective mode of the gluon field in strong interaction. We show that $\rho_{00}$ for the $\phi$ meson is a good analyzer for fields even if they may strongly fluctuate in space-time.