The kaon's 50th anniversary

17 December 1997

December 20, 1947, saw the publication of the first images capturing the decay of particles we now know as kaons - the first examples of "strange" particles. (We now know the kaons are mesons containing strange quarks.) The tell-tale tracks appeared in a cloud chamber exposed to cosmic rays, which was operated by Clifford Butler and George Rochester at the University of Manchester. The left-hand image shows the decay of a neutral kaon, captured the previous year, 1946. Being uncharged the neutral kaon leaves no track, but a "V" of tracks appears when it decays into two lighter charged particles, each of which is a pion (just below the central bar towards right of the chamber). The right-hand image shows the decay of a charged kaon into a muon and a neutrino. The kaon has come in at the top right of the chamber and the decay occurs where the track appears to bend to the left abruptly. The track beyond this kink is due to the muon, which penetrates the bar across the chamber. The neutrino has no charge and so remains invisible in the detector - its presence is inferred from the imbalance in momentum where the kink occurs. The decays of kaons are related to nuclear beta-decay, and occur through the weak force.

Credit: Robin Marshall, University of Manchester (R.Marshall@man.ac.uk)

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