Rigorous pion-pion scattering lengths from threshold πN → ππN data

A new evaluation of the universal ππ scattering length relation is used to extract the ππ s-wave scattering lengths from threshold pion production data. Previous work has shown that the chiral perturbation series relating threshold pion production to ππ scattering lengths appears to converge well only for the isospin-2 case, giving a2 = −0.031 ± 0.007mπ−1. A model-independent and data-insensitive universal curve then implies a0 = 0.235 ± 0.03mπ−1 for the isospin-0 scattering length.