Measurement of global spin alignment of vector mesons at RHIC

We report the measurements of spin alignment (${\rho }_{00}$) for $K^{\ast 0}$, $\overline{K^{\ast 0}}$, $K^{\ast +}$, and $K^{\ast -}$ vector mesons in RHIC isobar collisions (Zr+Zr and Ru+Ru) at $\sqrt{s_{\mathrm{N}\mathrm{N}}}$ = 200 GeV. We observe the first non-zero spin alignment for $K^{\ast \pm }$ in heavy-ion collisions. The $K^{\ast \pm }$ ${\rho }_{00}$ is about 3.9$\sigma$ larger than that of $K^{\ast 0}$. The observed difference and the ordering between $K^{\ast \pm }$ and $K^{\ast 0}$ are surprising, and require further inputs from theory. When comparing between the isobar and Au+Au collisions, no significant system size dependence in $K^{\ast 0}$ ${\rho }_{00}$ is observed within uncertainties.