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

We have measured total cross sections for the reaction ${\mathrm{\pi }}^{+}$p→${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$p at incident pion kinetic energies of 190, 200, 220, 240, and 260 MeV. We use this result to deduce a new value of the chiral symmetry breaking parameter, ξ=-0.25±0.10, in a global constrained fit of the five ππN near-threshold amplitudes. Consequently, we report new soft pion model values for the s-wave ππ scattering lengths.