Stereoscopic photographs showing an unusual fork (a b) in the gas. The direction of the magnetic field is such that a positive particle coming downwards is deviated in an anticlockwise direction (Image: Nature)
Butler and Rochester discovered the kaon – the first strange particle – in an experiment using a cloud chamber. They took two photos – one of two cloud chamber photographs – one of them seemed to be a charged particle decaying into a charged particle and something neutral. The estimated mass of the particle was roughly 200 times that of the proton.
Their paper, Evidence for the existence of new unstable elementary particles, noted:
Among some fifty counter-controlled cloud-chamber photographs of penetrating showers which we have obtained during the past year as part of an investigation of the nature of penetrating particles occurring in cosmic ray showers under lead, there are two photographs containing forked tracks of a very striking character. These photographs have been selected from five thousand photographs taken in an effective time of operation of 1500 hours. On the basis of the analysis given below we believe that one of the forked tracks represents the spontaneous transformation in the gas of the chamber of a new type of uncharged elementary particle into lighter charged particles, and that the other represents similarly the transformation of a new type of charged particle into two light particles, one of which is charged and the other uncharged
Read more: "Evidence for the existence of new unstable elementary particles" G. D. Rochester & C. C. Butler, Nature 160 (1947) 855-857