Proton-Proton Interactions at 5.5 GeV/<i>c</i>

This report is based on about 10 500 $\mathrm{pp}$ collision events produced in the 81-cm Saclay hydrogen bubble chamber at CERN. Cross-section values for the different identified final states and resonances are given. The isobars ${{N}^{*}}_{1238}$, ${{N}^{*}}_{1420}$, ${{N}^{*}}_{1518}$, ${{N}^{*}}_{1688}$, ${{N}^{*}}_{1920}$, and ${{N}^{*}}_{2360}$ were identified and their production cross-section values were found via a best-fit analysis of different invariant-mass histograms. About 70% of the isobars are connected with the quasi-two-body reactions $\mathrm{pp}\ensuremath{\rightarrow}{N}^{*}N$ and $\mathrm{pp}\ensuremath{\rightarrow}{N}^{*}{N}^{*}$. The reaction $\mathrm{pp}\ensuremath{\rightarrow}n{{N}^{*}}_{1238}(p{\ensuremath{\pi}}^{+})$ with a cross section of 3.25\ifmmode\pm\else\textpm\fi{}0.16 mb was analyzed in terms of a peripheral absorption model, which was found to be in good agreement with the data. Various decay modes of the ${{N}^{*}}_{1518}$ and ${{N}^{*}}_{1688}$ isobars were observed and their branching ratios determined. The branching ratio of $n{\ensuremath{\pi}}^{+}$ to $p{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ was found to be 0.77\ifmmode\pm\else\textpm\fi{}0.45 for ${{N}^{*}}_{1518}$ and 0.67\ifmmode\pm\else\textpm\fi{}0.40 for ${{N}^{*}}_{1688}$. The branching ratio of ${{N}^{*}}_{1238}(p{\ensuremath{\pi}}^{+}){\ensuremath{\pi}}^{\ensuremath{-}}$ to $p{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ of ${{N}^{*}}_{1688}$ was estimated to be 0.74\ifmmode\pm\else\textpm\fi{}0.14. Pion production turned out to be mainly due to decay of isobars. Production of meson resonances turned out to be less important; the reaction $\mathrm{pp}\ensuremath{\rightarrow}\mathrm{pp}{\ensuremath{\omega}}^{0}\ensuremath{\rightarrow}\mathrm{pp}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$ was identified with a cross-section value of 0.11\ifmmode\pm\else\textpm\fi{}0.02 mb. Finally, the production of neutral strange particles with a cross section of 0.45\ifmmode\pm\else\textpm\fi{}0.04 mb is descussed. Strong formation of ${{Y}^{*}}_{1385}$ is observed.