π−−pInteractions at 905, 960, and 1100 MeV

Single-pion production in ${\pi }^{-}-p$ interactions has been studied at 905, 960, and 1100 MeV. Comparison with the isobar and one-pion-exchange (OPE) mechanisms of pion production shows that, below 1 BeV, pion production occurs primarily through the formation of an intermediate excited state of the nucleon (isobar), while at higher energies the influence of the $\rho$ resonance in the $\pi \pi$ system becomes increasingly important. There is some evidence for an $I=2\mathrm{}$ state in the events at the lower energies.