Large-Nc QCD and Low Energy Interactions

I shall first explain how, in the Standard Theory, the coupling constants of the chiral Lagrangians of strong and electroweak interactions of Goldstone particles at low energies, are related to QCD Green's functions of color singlet operators. I shall then show how these relations, when formulated within the framework of the $1/N_c$--expansion, offer a systematic approach to calculate low energy observables. I shall illustrate this with several examples: the electroweak $\pi^{+}-\pi^{0}$ mass difference, the $\pi^{0}\rightarrow e^{+}e^{-}$ and $\eta\rightarrow \mu^{+}\mu^{-}$ decay rates, the hadronic vacuum polarization contribution to the muon $g-2$, the $K\rightarrow \pi\pi$ amplitudes induced by electroweakpenguin--type'' operators, and $K^{0}-\bar{K}^{0}$ mixing.
hep group