Unusualπ−μDecays in Photographic Emulsions

A total of 11 841 $\pi -\mu$ decays has been studied in Ilford C-2 and G-5 photographic plates, where both the $\pi$- and $\mu$-meson tracks stopped in the emulsion. In 12 cases the range of the $\mu$-meson track from the $\pi$-meson decay is less than 480 microns and in 2 cases the range is greater than 720 microns. The ranges of the $\mu$-meson tracks of the 14 unusual $\pi -\mu$ decays are: 120, 185, 258, 260, 290, 416, 430, 441, 444, 470, 470, 476, 828, and 1035 microns. The normal range of the $\mu$-meson from $\pi$-meson decays is about 600 microns. A study of the grain density in C-2 plates, gap density in G-5 plates, and the number and energy of the $\delta$-rays along the short $\mu$-meson tracks indicates that the $\mu$-mesons were ejected from the $\pi$-decays with a lower velocity than from normal $\pi -\mu$ decays. It is possible to explain the short range of the $\mu$-meson in 4 of the events and the long range of the $\mu$-meson in the 2 events by assuming that the $\pi$-meson decayed in flight. The grain density or gap density along the $\mu$- and $\pi$-meson tracks in the remaining 8 events is inconsistent with the assumption that the short range of the $\mu$-meson is due to the decay in flight of the $\pi$-meson. If a correction is made for the thickness of the emulsion, the probability that a $\mu$-meson from a stopped $\pi$-meson decay will have a range less than 480 microns is found to be 3.3±1.3×${10}^{-4\mathrm{}}$. This probability is in general agreement with theoretical predictions based upon the assumption that a soft photon is occasionally emitted from the $\pi -\mu$ decay resulting from the charge acceleration of the $\mu$-meson.