Reactionπ+p→pπ+π+π−π0at18.5 GeVc

The reaction ${\pi }^{+}p\to p{\pi }^{+}{\pi }^{+}{\pi }^{-}{\pi }^0$ has been studied at an incident ${\pi }^{+}$ momentum of $18.5 \frac{\mathrm{GeV}}{c}$. This reaction is dominated by strong resonance production and low four-momentum transfer squared from the target to the outgoing proton. A large excess of events in the ${\pi }^{+}{\pi }^{+}{\pi }^{-}{\pi }^0$ effective-mass distribution in the region from 1.52 to 2.32 GeV is observed. This enhancement is attributed primarily to $p{\rho }^{+}{\rho }^0$ and $\mathrm{pA}\pi$ final states. A study of angular distributions in the $p{\rho }^{+}{\rho }^0$ final state suggests the importance of a double-peripheral production mechanism for these events.