Trapping of negative kaons by metastable states during the atomic cascade in liquid helium

Abstract

We observed two distinct peaks, 205-MeV/c ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ and 235-MeV/c ${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$, associated with ${K}_{\ensuremath{\pi}2}^{\mathrm{\ensuremath{-}}}$ and ${K}_{\ensuremath{\mu}2}^{\mathrm{\ensuremath{-}}}$ decays at rest, respectively, from negative kaons stopped in liquid helium. These peaks were found to be delayed with respect to the stopping ${K}^{\mathrm{\ensuremath{-}}}$, showing that stopped ${K}^{\mathrm{\ensuremath{-}}}$ mesons of about 2% fraction are trapped in metastable states with an overall lifetime of about 40 nsec. This observation provides a direct evidence for Condo's trapping hypothesis for the at-rest decay components of ${K}^{\mathrm{\ensuremath{-}}}$ and ${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ in liquid helium.