π^{+}p Interactions at 4 GeV/c

Abstract

In photographs taken with the Saclay 81-cm hydrogen bubble chamber at the CERN proton synchrotron about 16 000 ${\pi }^{+}p$ scatterings at $4 \frac{\mathrm{GeV}}{c}$ with two and four charged secondaries have been analyzed and identified. The cross sections for the individual two-and four-prong channels and for the production of resonances are given. The various effective-mass distributions are shown. The reaction ${\pi }^{+}p\to p{\pi }^{+}{\pi }^0$ is dominated by $N^{*++}$ and ${\rho }^{+}$ production. From the reaction ${\pi }^{+}p\to n{\pi }^{+}{\pi }^{+}$ the $T=2\mathrm{} \pi \pi$ cross section as a function of the energy has been deduced applying the Chew-Low and the Selleri formula. The $\pi \pi$ angular distribution is isotropic ($s$ wave) up to ∼800 MeV. In the reaction ${\pi }^{+}p\to p{\pi }^{+}\left(m{\pi }^0\right)$ the $N^{*++}$ and the ${A_2}^{+}$ occur. An estimate of the decay ratio $A_2\to \frac{\eta \pi }{A_2}\to \rho \pi$ yielded 0.3±0.2. About 75% of the reactions ${\pi }^{+}p\to p{\pi }^{+}{\pi }^{+}{\pi }^{-}$ proceed via $N^{*++}$ and ${\rho }^0$ production. The major features of this reaction can be explained by the one-pion-exchange (OPE) model; the various distributions have been computed by a Monte Carlo method applying the Ferrari-Selleri off-shell corrections to the OPE formula. The experimental distributions can be reproduced rather well by the model. In the channel ${\pi }^{+}p\to N^{*++}{\pi }^{+}{\pi }^{-}$ the angular-momentum state of the ${\pi }^{+}{\pi }^{-}$ system has been studied. A highly pure sample of the reaction ${\pi }^{+}p\to N^{*++}{\rho }^0$ has been obtained by limitation to small momentum transfers. A weak correlation between the $N^{*++}$ and ${\rho }^0$ decay angles is observed. In the ${\rho }^0{\pi }^{+}$ effective-mass distribution the $A_1$ and $A_2$ show up. The $A_1$ and $A_2$ have a different ${\Delta }^2$ and decay angular distribution; the $A_1$ enhancement can perhaps be explained by the OPE model. In the $p{\pi }^{+}{\pi }^{-}$ effective-mass distribution the $T=\frac{1}{2}$ isobars at 1518 and 1688 MeV show up; there is no definite enhancement in the $p{\pi }^{+}{\pi }^{+}$ mass distribution. From the reactions ${\pi }^{+}p\to p{\pi }^{+}{\pi }^{+}{\pi }^{-}{\pi }^0$ a highly pure sample of the channel ${\pi }^{+}p\to N^{*++}{\omega }^0$ could be obtained by limitation to small ${\Delta }^2$. The $B$ meson is present in the ${\omega }^0{\pi }^{+}$ mass distribution; no definite conclusion for the spin and parity of the $B$ meson could be reached. The previously observed enhancement at 975 MeV in the $\rho \pi$ mass distribution is discussed further. In the reaction ${\pi }^{+}p\to n{\pi }^{+}{\pi }^{+}{\pi }^{+}{\pi }^{-}$ and in the four-prong reactions with several neutrals there is no strong resonance production except for a slight amount of $N^{*}$ and $\rho$. Finally the ${\Delta }^2$ distributions for six two-body reactions (elastic scattering and five channels with resonance production) are given and compared with each other. In all distributions a small accumulation of events at high ${\Delta }^2$ is observed besides the strong peak at small ${\Delta }^2$.