π−+p→π+π+NucleonReactions

Abstract

Data from two ${\pi }^{-}$+proton experiments, at 2.75 and 3.0 GeV/c, have been compared and combined. Some 3400 events of the final-state ${\pi }^{-}{\pi }^{+}$ neutron and 2000 of ${\pi }^{-}{\pi }^0$ proton were found. These events are dominated by the $\rho$ resonance. The decay angular distribution of the ${\rho }^{-}$ agrees surprisingly well with the one-pion-exchange theory modified for absorption effects, as calculated by Gottfried and Jackson. The $\rho$-production angular distributions exhibit a difference between ${\rho }^{-}$ and ${\rho }^0$ at ${\Delta }^2$ values (4-momentum transfer to nucleon, squared) of 20 to 40 ${M_{\pi }}^2·{\rho }^0$ production shows a weak backward peak. The ${\rho }^0$ mass region shows effects interpretable as due to the 720-MeV $T=0\mathrm{}$ $s$-wave $\pi \pi$ resonance, and to ${\omega }^0\to {\pi }^{+}{\pi }^{-}$ decay. Analysis of the data in terms of $\pi \pi$ scattering is discussed. A simple phase-shift analysis using "physical-region" single-pion-production data directly, without extrapolation to the one-pion pole, will not in general be valid for ${\Delta }^2$ greater than 2 or 3 ${M_{\pi }}^2$; insufficient information exists to determine whether data of lower ${\Delta }^2$ values can be meaningfully used without extrapolation. $S$-wave $\pi \pi$ scattering in the mass region below the $\rho$ is discussed. The ABC and $\sigma$ effects do not show up in these data.