We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of Λ5He and Λ12C 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 (Λ5He) = 0.45 ± 0.11 (stat) ± 0.03 (syst) in the kinematic region of cos θNN < -0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of Γ(Λp → nn)/Γ (Λp → np). The Γn/Γp ratio was measured also for the NMWD of Λ12C. It is also close to 0.5. Those ratios are consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture.
ASJC Scopus subject areas
- Nuclear and High Energy Physics