$\rho^0-\omega$ mixing in U(3)}$_L\times$ U(3)$_R$ chiral theory of   mesons
  In this paper, U(3)$_L\times$ U(3)$_R$ chiral theory of mesons is extended to the leading order in quark mass expansion in order to evaluate the $\rho^0-\omega$ mixing. It is shown that the use of path integral method to integrate out the quark fields naturally leads to the $\rho^0-\omega$ mixing vertices, and this mixing is entirely from the quark loop in this theory. The off-shell behaviour of the mixing amplitude is analyzed. The on-shell mixing amplitude is obtained from the decay $\omega\to \pi^+\pi^-$. Furthermore, the constraints on the light quark mass parameters are extracted from the $\rho^0-\omega$ mixing and the mesons masses, and the mass splitting of $K^*(892)$-mesons is predicted. 
Dao-Neng Gao and Mu-Lin Yan (USTC)