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

We propose that a significant positive deviation from 1/3 for the spin density matrix element ρ00 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 ρ00 are much larger than 1/3, while the data of Λ and Λ¯ polarization seem not to allow such a significant and positive deviation. The contribution is from the polarization of the strange quark and antiquark through the spin-orbit interaction in the ϕ field, a similar interaction that is responsible for nuclear shell structure at the nucleon level. We show that ρ00 for the ϕ meson is a good analyzer for fields even if they may strongly fluctuate in space-time.