${\mathrm{\pi }}^{\mathrm{-}}$p→${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$n near threshold and chiral symmetry breaking

Total cross sections, angular, and mass distributions for the reaction ${\mathrm{\pi }}^{\mathrm{-}}$p→${\mathrm{\pi }}^0$ ${\mathrm{\pi }}^0$n have been measured for ${\mathit{p}}_{\mathrm{\pi }}^{\mathrm{-}}$(lab)=7–140 MeV/c above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, ξ, of -0.98±0.52. The ππ scattering lengths ${\mathit{a}}_{\mathit{I}}$ for isospin I=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are ${\mathit{a}}_0$=(0.207±0.028)${\mathit{m}}_{\mathrm{\pi }}^{\mathrm{-}1}$ and ${\mathit{a}}_2$=(-0.022±0.011)${\mathit{m}}_{\mathrm{\pi }}^{\mathrm{-}1}$. These values are consistent with chiral symmetry broken by the Weinberg ππ interaction and the effects of the ${\mathit{f}}_0$(975) scalar meson.