Relativistic spin dynamics for vector mesons

We propose a relativistic theory for spin density matrices of vector mesons based on Kadanoff-Baym equations in the closed-time-path formalism. The theory puts the calculation of spin observables such as the spin density matrix element $ρ_{00}$ for vector mesons on a solid ground. Within the theory we formulate $ρ_{00}$ for $φ$ mesons into a factorization form in separation of momentum and space-time variables. We argue that the main contribution to $ρ_{00}$ at lower energies should be from the $φ$ fields that can polarize the strange quark and antiquark in the same way as electromagnetic fields. The key observation is that there is correlation inside the $φ$ meson wave function between the $φ$ field that polarizes the strange quark and that polarizes the strange antiquark. This is reflected by the fact that the contributions to $ρ_{00}$ are all in squares of fields which are nonvanishing even if the fields may strongly fluctuate in space-time. The fluctuation of strong force fields can be extracted from $ρ_{00}$ of quarkonium vector mesons as links to fundamental properties of quantum chromodynamics.