# 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
Last modified: Mon Oct 29 15:17:21 EST 2001