Vector Meson Dominance and Rho-Omega Mixing
  The scale of a phenomenologically successful charge-symmetry-violating nucleon-nucleon interaction, that attributed to meson exchange with a $\Delta I =1$ rho-omega transition, is set by the Coleman-Glashow SU(2) breaking tadpole mechanism. A single tadpole scale has been obtained from symmetry arguments, electromagnetic meson and baryon measured mass splittings, and the observed isospin violating ($\Delta I=1$) decay $\omega \to \pi^+\pi^-$. The hadronic realization of this tadpole mechanism lies in the $I = 1$ $a_0$ scalar meson. We show that measured hadronic and two-photon widths of the $a_0$ meson, with the aid of the Vector Meson Dominance model, recover the universal Coleman-Glashow tadpole scale. 
S. A. Coon (New Mexico State University), M. D. Scadron (University of   Arizona)