Eur. Phys. J. A (2007) 33: 259-263
https://doi.org/10.1140/epja/i2007-10470-x

HYP 2006

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

¹ Department of Physics, Seoul National University, 151-747, Seoul, Korea
² Department of Physics, Osaka University, 560-0043, Toyonaka, Japan
³ High Energy Accelerator Research Organization (KEK), 305-0801, Tsukuba, Japan
⁴ Laboratory of Physics, Osaka Electro Communication University, 572-8530, Neyagawa, Japan
⁵ Department of Physics, Tohoku University, 980-8578, Sendai, Japan
⁶ Korea Research Institute of Standards and Science (KRISS), 305-600, Daejeon, Korea
⁷ Department of Physics, University of Tokyo, 113-0033, Hongo, Japan
⁸ Department of Physics, Tokyo Institute of Technology, 152-8551, Ookayama, Japan
⁹ Gesellschaft für Schwerionenforschung mbH (GSI), 64291, Darmstadt, Germany
¹⁰ Department of Physics, Ewha Womans University, 120-750, Seoul, Korea
¹¹ RIKEN Wako Institute, RIKEN, 351-0198, Wako, Japan

^* e-mail: bhang@phya.snu.ac.kr

Received: 28 February 2007
Accepted: 7 July 2007
Published online: 3 September 2007

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 5lam and ¹² _Λ 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 ¹² _Λ 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.