Multiple-Pion Production by 2-BeVπ−

A report is given of a study of pion-proton interactions in a liquid-hydrogen bubble chamber. A 2.1-BeV/c negative pion beam incident upon a 14-in.-diam chamber caused interactions from which those with four charged secondaries were selected and measured. Fits were attempted to the following final state: $3\pi +p$, $3\pi +p+{\pi }^0$, and $4\pi +n$. By normalizing to a total cross section of 35.7 mb, the partial cross sections for the above-listed final states are found to be 1.67±0.08, 1.17±0.06, and 0.37±0.02 mb. The $3\pi +p$ reaction was found to be dominated by the production of the $\frac{3}{2}-\frac{3}{2}$ pion-nucleon isobar. The cross section for this process is 0.53±0.05 mb. Evidence for the eta and omega mesons was found in the $3\pi +p+{\pi }^0$ final state. Their cross sections are 0.037±0.015 and 0.26±0.035 mb, respectively. A Dalitz plot for eta decays is presented. Evidence is given for a double peak in the neutral two-pion mass spectrum for $3\pi +p$ and $4\pi +n$ events. The peaks coincide with those attributed to the rho meson in other experiments. The second of the two peaks corresponds well with the mass of the ${\omega }^0$, A branching ratio of ${\omega }^0\to \frac{2\pi }{{\omega }^0}\to 3\pi$ is found to be >0.1. An attempt is made to calculate the pion-pion cross section by isolating events with low momentum transfer to the isobar. In the various processes, enhancement is found in the nucleon $+2\mathrm{}\pi$ mass spectra corresponding to the $T=\frac{3}{2}$ state of mass 1920 MeV. It is proposed that the elementary crossed diagram can explain these effects.