V. E. Barnes et al., Phys. Rev. Lett. 12, 204 (1964)
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The Xi or "cascade" baryon was seen in the same reaction which led to the discovery of the omega-minus baryon at Brookhaven in 1964. At left is a sketch of the bubble chamber photograph in which the omega-minus baryon was discovered. The decay of the Xi indicated in the diagram is
In this decay of the Xi particle, a lambda baryon and two gamma rays are shown as the products. Actually, the primary decay of the Xi is to a lambda-zero and a pi-zero meson. This cannot be a purely electromagnetic decay because strangeness is not conserved. Also, it's lifetime in the range of 10-10 seconds puts it in the common range for weak-interaction decays. It is just that the π0 decays into two gammas with a lifetime of about 10-16 seconds (an electromagnetic decay), so the two gamma rays appear to come from the same point as the lambda.
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