Multiple Pion Production by Pions and the Isobar Model

Multiple pion production by pions in hydrogen has been studied in the interaction ${\pi }^{-}+p\to p+{\pi }^{-}+{\pi }^{+}+{\pi }^{-}$ at 810, 970, and 1100 Mev. Total cross sections were measured as 0.26±0.14 mb at 810 Mev, 0.32±0.21 mb at 970 Mev, and 0.54±0.32 mb at 1100 Mev. The kinematics of this pion production reaction at these energies appear to be reasonably well described by phase space dependence, but provide some support for an "isobar cascade" scheme in which the $T=\frac{1}{2}$ "second resonance" is excited and decays to the $T=J=\frac{3}{2}$ isobar, which decays in turn to a final state consisting of a nucleon, a recoil pion, and two decay pions.