A Study of the proton spectra following the capture of K- in Li-6 and C-12 with FINUDA

Abstract Momenta spectra of protons emitted following the capture of K − in 6 Li and 12 C have been measured with 1% resolution. The 12 C spectrum is smooth whereas for 6 Li a well defined peak appears at about 500 MeV / c . The first observation of a structure in this region was identified as a strange tribaryon or, possibly, a K ¯ -nuclear state. The peak is correlated with a π − coming from Σ − decay in flight, selected by setting the π momentum larger than 275 MeV / c . The Σ − could be produced, together with a 500 MeV / c proton, by the capture of a K − in a deuteron-cluster substructure of the 6 Li nucleus. The capture rate for such a reaction is ( 1.62 ± 0.23 stat −0.44 +0.71 ( sys ) ) % / K stop − , in agreement with the existing observations on 4 He targets and with the hypothesis that the 6 Li nucleus can be interpreted as a ( d + α ) cluster.