Decay Asymmetry of Cosmic Ray Muons

Abstract

In this paper an account is given of the investigations on the decay asymmetry of cosmic-ray muons carried out in two different energy bands with the help of a multiplate cloud chamber. Although similar investigations have been made in different laboratories with scintillation counters, employing the delayed coincidence technique for the detection of the decay electrons, the cloud chamber has been chosen for some special advantages which are discussed in the paper. The most important among these are (i) the decay electrons are detected from zero delay up to a delay of about 50 milliseconds, which is never possible in the delayed coincidence method, (ii) the change of direction of the decay electrons in the absorbing material due to scattering is avoided by dividing the whole absorber into thin plates separated by gaps in which the electrons are observed. Even if these electrons are subsequently scattered their original direction of emission is clearly observed, as shown in figure 1 (a) and (b). After careful scanning of about 36 000 photographs taken by means of a fourfold coincidence and anticoincidence system about 1500 unambiguous μ-e decay events were selected. Of these, 1021 events correspond to a muon momentum interval 0.30-0.40 GeV/c and 482 events to a momentum interval 1.3-1.4 GeV/c at the experimental station, which was situated at a height 2.2 km. The up-down ratios of electrons emitted in the two momentum intervals are 1.17 ± 0.05 and 1.18 ± 0.08 respectively, which yield partial longitudinal polarizations of 0.34 ± 0.09 and 0.36 ± 0.11 after correcting for the depolarization effect of the atmosphere and moderator. Our results, therefore, show no evidence for the increase in the degree of polarization with increase of momentum of the muons which some workers have observed. The absolute magnitude of the polarization observed is consistent with a K/π ratio of about 5% in the pion spectral region 0.2 to 5.6 GeV.