WHAT PI-PI-SCATTERING TELLS US ABOUT CHIRAL PERTURBATION-THEORY

We describe a rearrangement of the standard expansion of the symmetry-breaking part of the QCD effective Lagrangian that includes into each order additional terms which in the standard chiral perturbation theory (chiPT) are relegated to higher orders. The new expansion represents a systematic and unambiguous generalization of the standard chiPT, and is more likely to converge rapidly. It provides a consistent framework for a measurement of the importance of additional ''higher order'' terms whose smallness is usually assumed but has never been checked. A method of measuring, among other quantities, the QCD parameters m[qqBAR] and the quark mass ratio m(s)/m is elaborated in detail. The method is illustrated using various sets of available data. Both of these parameters might be considerably smaller than their respective leading-order standard chiPT values. The importance of new, more accurate, experimental information on low-energy pi-pi scattering is stressed.