On the mean life time of K-mesons produced by the cosmic radiation

Abstract

An experiment performed at Echo Lake (Colorado) in order to measure the mean lifetime of charged heavy mesons produced by the cosmic radiation is described. The experimental procedure consists in the electronic measurement (by means of a « chronotron ») of the time lag between the occurrence of a nuclear interaction and the delayed emission of a fast charged secondary in the decay (at rest) of an unstable particle produced in the interaction. The nuclear event is detected through its penetrating secondaries by means of a large scintillation counter and a Geiger counters hodoscope, whereas the decay secondary is detected by means of a large directional water Cerenkov counter. The experimental arrangement is described in Sect.2; the timing errors and the criteria used to select and classify the events are discussed in Sect.3. The delay distributions obtained are analyzed in Sect. 4: they can be resolved into a short life component (lifetime (9.6 ± 0.810-9) s) and a long life component (lifetime (2.2 ± 0.2) μs); the latter is interpreted as due to the electrons of the composite π→μ→e decay of π+-mesons produced locally by the incident nucleons. The possibility that the short life component is due to spurious effects of some kind is discussed and ruled out in Sect. 5 on the basis of the results of auxiliary measurements: the conclusion is that the lifetime obtained should be associated essentially with the Kμ2 decay mode, with a possible small admixture of Kπ2(θ+). A rough estimate of the production ratio of such particles to positive π-mesons is made, taking into account the corresponding detection efficiencies; the value found is rather higher than those (1:30- i : 70) obtained from previous cosmic ray experiments. The lifetime obtained is compared with the recent results of other Authors and the assumptions of a purely exponential decay curve with an unique lifetime is discussed.