In a study of 2.7 million $\frac{J}{\psi }$ decays with the Mark III, a new magnetic detector at SPEAR, we have observed the sequential decay $\frac{J}{\psi }\to \gamma {\eta }_c$, ${\eta }_c\to \phi \phi$. The product branching fraction is (1.02±0.25±0.14)×${10}^{-4\mathrm{}}$, where the quoted errors are statistical and systematic, respectively. Analysis of the final-state angular distributions provides the first experimental determination that the spin and parity of the ${\eta }_c$ are $0^{-}$.

• Received 9 April 1984

DOI:https://doi.org/10.1103/PhysRevLett.52.2126