Proton-Proton Interactions at 5.5 GeV/c

Abstract

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 $\mathrm{pp}\to N^{*}N$ and $\mathrm{pp}\to N^{*}{N}^{*}$. The reaction $\mathrm{pp}\to n{N^{*}}_{1238}\left(p{\pi }^{+}\right)$ with a cross section of 3.25±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 $n{\pi }^{+}$ to $p{\pi }^{+}{\pi }^{-}$ was found to be 0.77±0.45 for ${N^{*}}_{1518}$ and 0.67±0.40 for ${N^{*}}_{1688}$. The branching ratio of ${N^{*}}_{1238}\left(p{\pi }^{+}\right){\pi }^{-}$ to $p{\pi }^{+}{\pi }^{-}$ of ${N^{*}}_{1688}$ was estimated to be 0.74±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 $\mathrm{pp}\to \mathrm{pp}{\omega }^0\to \mathrm{pp}{\pi }^{+}{\pi }^{-}{\pi }^0$ was identified with a cross-section value of 0.11±0.02 mb. Finally, the production of neutral strange particles with a cross section of 0.45±0.04 mb is descussed. Strong formation of ${Y^{*}}_{1385}$ is observed.