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

We observed two distinct peaks, 205-MeV/c π^- and 235-MeV/c μ^-, associated with K_π2^- and K_μ2^- decays at rest, respectively, from negative kaons stopped in liquid helium. These peaks were found to be delayed with respect to the stopping K^-, showing that stopped K^- 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^- and π^- in liquid helium.