Spin Alignment of Vector Mesons Induced by Local Spin Density Fluctuations

We propose a novel mechanism that the spin alignment of vector meson $\phi$ and $K^{\ast 0}$ in most central heavy ion collisions is induced by intrinsic QCD dynamics without external vortical/ magnetic fields. The local spin density fluctuation of quarks due to the axial-vector and tensor interactions between quarks can induce the spin alignments of vector meson. It is found that axial-vector interaction results in ${\rho }_{00}<1\mathrm{/}3$ for the spin alignment of $\phi$ meson, while tensor interaction results in ${\rho }_{00}>1\mathrm{/}3$. We argue that the spin alignment of $K^{\ast 0}$ due to the spin density fluctuation from flavor-mixing would be a signature or hint of instanton.