Nucleon-nucleon coincidence measurement in the non-mesonic weak decay of HeΛ5 and CΛ12 hypernuclei

We have measured both yields of neutron-proton and neutron-neutron pairs emitted from the non-mesonic weak decay process of HeΛ5 and CΛ12 hypernuclei produced via the (π+,K+) reaction for the first time. We observed clean back-to-back correlation of the np- and nn-pairs in the coincidence spectra for both hypernucLei. The ratio of those back-to-back pair yields, Nnn/Nnp, must be close to the ratio of neutron- and proton-induced decay widths of the decay, Γn(Λn→nn)/Γp(Λp→np). The obtained ratios for each hypernuclei support recent calculations based on short-range interactions.