Identify the origin of vector-meson spin alignment

Identify the origin of the unexpectedly large deviations of vector-meson spin-alignment observables from $\rho_{00}=1/3$ in light and charm mesons produced in relativistic heavy-ion collisions.

Background

The $\rho_{00}$ element of the spin-density matrix for vector mesons such as the phi, K-star, J/psi, and D-star has been measured at RHIC and the LHC. The observed deviations from the value $1/3$, corresponding to equally populated spin states, are much larger than those expected from a global-vorticity scenario based on quark polarization and coalescence. The paper therefore identifies the physical origin of vector-meson spin alignment as a major unresolved issue.