Isobar Production Mechanisms in Proton-Proton Collisions at 5 GeV/c

Abstract

Approximately 7500 four-prong proton-proton collisions at 5 GeV/c have been studied using the BNL 80-in. hydrogen bubble chamber. The three principal reactions studied with their cross sections are: $(a) p+p\ensuremath{\rightarrow}p+p+{\ensuremath{\pi}}^{+}+{\ensuremath{\pi}}^{\ensuremath{-}} 2.96\ifmmode\pm\else\textpm\fi{}0.12 \mathrm{mb},$ $(b) p+p\ensuremath{\rightarrow}p+p+{\ensuremath{\pi}}^{+}+{\ensuremath{\pi}}^{\ensuremath{-}}+{\ensuremath{\pi}}^{0} 1.76\ifmmode\pm\else\textpm\fi{}0.07 \mathrm{mb},$ $(c) p+p\ensuremath{\rightarrow}p+n+{\ensuremath{\pi}}^{+}+{\ensuremath{\pi}}^{+}+{\ensuremath{\pi}}^{\ensuremath{-}} 2.19\ifmmode\pm\else\textpm\fi{}0.09 \mathrm{mb}.$ In each reaction both single and double ${N}^{*}$ production occurs and the decay angular distributions and density matrix elements have been calculated whenever possible. These parameters indicate that pseudo-scalar (pion) exchange appears to be dominant at low values of momentum transfer and that at higher momentum transfer absorptive effects become important. We find no evidence for $\ensuremath{\rho}$ exchange in the reaction $p+p\ensuremath{\rightarrow}{N}^{*++}+{N}^{*0}$. The formation of the $\ensuremath{\eta}$- and $\ensuremath{\omega}$-meson resonances is observed in reaction (b), but in none of the reactions studied is any significant $\ensuremath{\rho}$-meson production found to occur. In reaction (c) we find a deviation from phase space in the region of the 1580-MeV mass bump in the $p{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}$ ($I=\frac{5}{2}$) system which was observed by Alexander et al., but we ascribe this departure from phase space at our energy as being principally due to ${N}^{*++}$ and ${N}^{*\ensuremath{-}}$ formation in this channel.