The quenching of nucleon yields in the nonmesonic weak decay of Λ -hypernuclei and the three-body weak decay process

Abstract

Recent exclusive coincidence measurements of non-mesonic weak decays (NMWD) reported for the ratio of the partial decay width of neutron-induced-to-proton-induced NMWD, Γ n/Γ p , values of 0.45±0.11±0.03 and 0.51±0.13±0.04 for Λ 5 He and Λ 12 C, respectively. These observations agree well with the improved theoretical Γ n /Γ p ratios which are in the range of 0.3–0.7. It appears that the long-standing discrepancy between the experimental and theoretical values of Γ n /Γ p has finally been solved. However, when compared to the results of intra-nuclear cascade (INC) calculations, the observed numbers of both single nucleons and coincident nucleon pairs are strongly quenched. The quenching of the proton yield observed previously has been interpreted as an increase of the Γ n /Γ p ratio. On the other hand, significant contributions from the two-nucleon-induced three-body process ΛNN → nNN are predicted. Indeed, the angular correlation of the emitted nucleon pairs in the NMWD of Λ 12 C showed not only decay events in back-to-back kinematics, but also events with non-back-to-back kinematics. In this paper we show that the difficulties to extract the correct Γ n /Γ p ratio from the proton spectra is related to the three-body weak-interaction process which strongly quenches the nucleon yields.