The Chiral Coupling Constants lb{1} and lb{2} from πpi Phase Shifts

A Roy equation analysis of the available $\pi\pi$ phase shift data is performed with the $I=0$ S- wave scattering length $a^0_0$ in the range predicted by the one-loop standard chiral perturbation theory. A suitable dispersive framework is developed to extract the chiral coupling constants \lb{1}, \lb{2} and yields \lb{1}$=-1.70\pm0.15$ and \lb{2}$\approx 5.0$. We remark on the implications of this determination to (combinations of) threshold parameter predictions of the three lowest partial waves.