Experimental Study of Double-Charged Pion Production in (π−,p) Collisions at 900 MeV

Abstract

Eight hundred and forty events of the kind ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}+p+{\ensuremath{\pi}}^{\ensuremath{-}}+{\ensuremath{\pi}}^{+}$ produced in the 20-in. BNL hydrogen bubble chamber by 900-MeV pions have been unambiguously identified using spatial reconstruction and kinematic fitting programs as well as ionization density estimates. The ${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$,${\ensuremath{\pi}}^{\ensuremath{-}}$ combined mass distributions can be fitted by smooth curves, with no deviation beyond statistical fluctuations; no indication has been found of any prominent pion-pion resonance in this interaction, which covers a mass range up to 610 MeV. The ${\ensuremath{\pi}}^{+}$,$p$ combined mass distribution differs markedly from the four-body phase-space curve, but can be well fitted by weighting the ${\ensuremath{\pi}}^{+}$,$p$ total cross-section curve at each point according to the amount of phase space available for production of an isobar of corresponding mass. Assuming that the interaction proceeds exclusively via formation of the (${\ensuremath{\pi}}^{+},p$) isobar, one can get a good fit to the ${\ensuremath{\pi}}^{\ensuremath{-}}$,${\ensuremath{\pi}}^{\ensuremath{-}}$ mass spectrum. This isobar model is also consistent with all of the observed angle and momentum distributions for both pions and protons. The momentum distributions show no indication of any pion-pion-proton resonance in the range up to 1550 MeV, or of any three-pion resonance in the range up to 750 MeV. The cross section for the events studied was measured and found to be (0.33\ifmmode\pm\else\textpm\fi{}0.04) mb.