  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.
