Hypernuclear weak decay experiments at KEK: n-n and n-p coincidence measurement

We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of 5ΛHe and 12ΛC formed via the (π+,K+) reaction. In both of n+p and n+n pair coincidence spectra, we observed a clean back-to-back correlation coming from the two-body decay of Λp→np and Λn→nn, respectively. We obtained the ratio of the nucleon pair numbers, Nnn/Nnp (5ΛHe) = 0.45 ± 0.11 (stat) ± 0.03 (syst) in the kinematic region of cosθNN<−0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of Γ(Λp→nn)/Γ(Λn→np). The Γn/Γp ratio was measured also for the NMWD of 12ΛC. It is also close to 0.5. Those ratios are consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture.