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

We observed two distinct peaks, 205-MeV/c ${\pi }^{\mathrm{-}}$ and 235-MeV/c ${\mu }^{\mathrm{-}}$, associated with $K_{\pi 2}^{\mathrm{-}}$ and $K_{\mu 2}^{\mathrm{-}}$ decays at rest, respectively, from negative kaons stopped in liquid helium. These peaks were found to be delayed with respect to the stopping $K^{\mathrm{-}}$, showing that stopped $K^{\mathrm{-}}$ 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 $K^{\mathrm{-}}$ and ${\pi }^{\mathrm{-}}$ in liquid helium.