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

We propose that a significant positive deviation from 1/3 for the spin density matrix element $ρ_{00}$ of the $ϕ$ meson may indicate the existence of a mean field of the $ϕ$ meson generated in heavy-ion collisions. This explains why STAR preliminary data for the $ϕ$ meson's $ρ_{00}$ are much larger than 1/3 while the data of $Λ$ and $\barΛ$ 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 $ϕ$ field, an effective mode of the gluon field in strong interaction. We show that $ρ_{00}$ for the $ϕ$ meson is a good analyzer for fields even if they may strongly fluctuate in space-time.

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