V0 particles

As with the π-mesons, all the early work on what have been called the ‘strange’ particles was done by means of cosmic-ray studies, using cloud chambers at sea level and at mountain altitudes, and using nuclear emulsions flown in high-altitude balloons. The first example of a particle other than those we have already discussed was reported by Leprince- Ringuet in 1944. A secondary cosmic-ray particle, which crossed the cloud chamber, produced a delta ray, or recoil electron, having substantial energy and emitted at a measurable angle. From the measured curvatures of the tracks in the magnetic field, and the scattering angle, it was possible to determine the mass of the incident particle, which was found to be 500 ± 50 MeV/c2. It is now clear that this particle must have been a Kmeson, but, at the time, when even the pion had not been identified, the significance of this single event was not clear.

The first clear examples of the new particles were observed in 1947 by Rochester and Butler at the University of Manchester. In these events the decays of the particles were observed, allowing a more convincing conclusion than any that could be obtained from observations on a single track, even when a delta ray was produced. Rochester and Butler operated a cloud chamber in a magnetic field and triggered it when an arrangement of Geiger counters near the chamber detected a penetrating cosmic-ray shower.