The Chiral Lagrangian parameters, ℓ1, ℓ2,

The all–important consequence of Chiral Dynamics for ππ scattering is the Adler zero, which forces ππ amplitudes to grow asymptotically. The continuation of this subthreshold zero into the physical regions requires a P –wave resonance, to be identified with the ρ. It is a feature of ππ scattering that convergent dispersive integrals for the I = 1 channel are essentially saturated by the ρ–resonance and are much larger than those with I = 2 quantum numbers. These facts predict the parameters ℓ 1 , ℓ 2 of the Gasser–Leutwyler Chiral Lagrangian, as well as reproducing the well–known KSFR relation and self-consistently generating the ρ−resonance.