We argue that the decay mode $\chi \to \psi \gamma {\pi }^0$ should be at least 700 times more important than $\chi \to \psi 3\pi$, and thus the upper limit on the branching fraction for $\chi \to \psi 3\pi$ is ∼0.1%. Assuming that the decay $\chi \to \psi \gamma {\pi }^0$ explains interesting features of the $\psi \gamma$ invariant-mass spectrum observed recently in the high-energy inclusive hadronic reaction $\pi p\to \psi \gamma X$, we deduce a rough estimate of the branching ratio $\frac{\Gamma (\chi \to \psi \gamma {\pi }^0)}{\Gamma (\chi \to \mathrm{all})}\simeq 10\%$. This appears to be particularly large for a Zweig-rule-forbidden process.

• Received 27 July 1981

DOI:https://doi.org/10.1103/PhysRevD.24.2019