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

Abstract

We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of 5 Λ He and 12 Λ C 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 Λ p → n p and Λ n → n n , respectively. We obtained the ratio of the nucleon pair numbers, N n n / N n p ( 5 Λ He) = 0.45 ± 0.11 (stat) ± 0.03 (syst) in the kinematic region of cos θ N N < − 0.8 . Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of Γ ( Λ p → n n ) / Γ ( Λ n → n p ) . The Γ n / Γ p ratio was measured also for the NMWD of 12 Λ C. It is also close to 0.5. Those ratios are consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture.