Unusualπ−μDecays in Photographic Emulsions

Abstract

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