Single-Isobar Production in Proton-Proton Interactions at 28.5 GeV/c

Two- and four-prong final states in a 28.5-GeV/c $p-p$ bubble-chamber experiment were examined for single-isobar production in various decay channels; results are compared with missing-mass-spectrometer data. The 1400 enhancement was found to be an $N\pi$ system with the ratio $\frac{n{\pi }^{+}}{p{\pi }^0}\sim 2$, which establishes the isospin assignment $I=\frac{1}{2}$. The final state ${\Delta }^{++}\mathrm{}\left(1238\right)n$ was measured and compared with low-energy data to show that the cross section decreases according to $P_{\mathrm{lab}}^{}{}_{}{}^{-(1.91\mathrm{}\pm 0.08)}$, consistent with single-pion exchange.