A cosmic-ray event is described in which a negative cascade particle and two neutral heavy mesons appear to be produced in a single nuclear interaction above a cloud chamber. It is suggested that this event may be an example of the associated production of a ${\Xi }^{-}$ particle with two ${\theta }^0$ particles according to the scheme of Gell-Mann.

• Received 11 July 1955

DOI:https://doi.org/10.1103/PhysRev.100.1457