Events have been selected by coincidences between separated shielded trays of Geiger counters and it is shown that they arise from three types of phenomena, namely, extensive air showers, casual coincidences, and particles travelling in almost a horizontal direction. Selection of the last type of event has enabled the absolute intensity to be determined at zenith angles between 88° and 90°. Using a π meson production spectrum which gives consistency with the established underground absorption curve the intensity of particles in the horizontal direction has been calculated on the assumption that π–μ decay is the only source of μ mesons. The calculated intensity agrees well with the observations. Alternatively, by comparing the rate in the horizontal direction with the known rate underground, a value of 4.5 ± 1.5 × 10−8 sec. is obtained for the lifetime of the parents of μ mesons which, because of the depth of the atmosphere in the horizontal direction (362 m.w.e.), must have energy exceeding 1011 ev. This suggests that at this energy the parents of μ mesons can be identified with the π meson, and the number of μ mesons arising from the decay of a shorter lived meson (e.g. K meson) must be relatively small.
Meson production in air showers and the search for light exotic particles