The7/11rule: An estimate ofmρ/fπ

We derive an estimate for the ratio of the rho mass and the pion decay constant from an analysis of vector and axial-vector two-point functions using large-$N_c,$ lowest-meson dominance, and operator product expansion, in the chiral limit. We discuss the extension of this analysis to the scalar and pseudoscalar sector. Furthermore, this leads to a successful parameter-free determination of the $L_i$ couplings of the chiral Lagrangian if an improved Nambu–Jona-Lasinio ansatz for Green functions is assumed at low energies.