Kinematically complete measurement of the reaction ${\mathrm{\pi }}^{\mathrm{-}}$p→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$n in the region of Δ dominance as a test of chiral Lagrangians

The first kinematically complete, good-statistics data for the (π,2π) reaction on the proton are presented. They involve double- and triple-differential cross sections as well as ${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$ angular correlations. At energies around the Δ resonance, they exhibit a marked difference from phase space. The data are qualitatively described within the framework of an extension of Weinberg’s effective chiral Lagrangian.