Photograph taken in the CERN/E.T.H Zurich 1.70 m—long Cloud Chamber during an experiment at the CERN synchrotron of 28.000 MeV.

The purpose of the experiment was to study mono- energetic $K^{0}_{2}$ kaons and especially to determine the branching ratio of the various decay modes. Protons accelerated by the synchrotron were directed on to a target and produced negative pions; "these interacted in a hydrogene target to give the $K^{0}_{2}$ kaons photographed by the cloud chamber installed in the synchrotron’ s North experimental hall. Clearly seen on the photograph is the end of a “V", the two branches of which are identified as, on the left, a positive pion and, on the right, an electron. Both of them as well as an invisible anti-neutrino are products of the $K^{0}_{2}$ decay. The momentum of the $K^{0}_{2}$ was 584 MeV/c. A large number of spirals confuse the photo. Those are due primarily to electrons captured by the magnetic field across the chamber, and produdsd by Compton effect to electron pairs produced by gamma rays associated with the beam.