Charge independence and symmetry are approximate symmetries of nature. The observations of the small symmetry breaking effects and the consequences of those effects are reviewed. The effects of the mass difference between up and down quarks and the off shell dependence $q^2$ of $rho^0$-$omega$ mixing are stressed. In particular, I argue that models which predict a strong $q^2$ dependence of $rho^0$-$omega$ mixing seem also to predict a strong $q^2$ variation for the $rho^0$-$gamma^*$ matrix element, in contradiction with experiment.
تحميل البحث