Kinematically complete measurement of the reaction pi- p ---> pi+ pi- n in the region of Delta dominance as a test of chiral Lagrangians

The first kinematically complete, good-statistics data for the ({pi},2{pi}) reaction on the proton are presented. They involve double- and triple-differential cross sections as well as {pi}{sup +}{pi}{sup {minus}} angular correlations. At energies around the {Delta} 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.