Nonmesonic Weak Decay of A-Hypernuclei within Independent-Particle Shell-Model

After a short introduction to the nonmesonic weak decay (NMWD)ΛN→nN of Λ-hypernuclei we discuss the long-standing puzzle on the ratio Γn/Γp, and some recent experimental evidences that signalized towards its final solution. Two versions of the Independent-Particle-Shell-Model (IPSM) are employed to account for the nuclear structure of the final residual nuclei. They are: (a) IPSM-a, where no correlation, except for the Pauli principle, is taken into account, and (b) IPSM-b, where the highly excited hole states are considered to be quasi-stationary and are described by Breit-Wigner distributions, whose widths are estimated from the experimental data. We evaluate the coincidence spectra in Λ4He, Λ5He, Λ12C, Λ16O, and Λ28Si, as a function of the sum of kinetic energies EnN = En+EN for N = n, p. The recent Brookhaven National Laboratory experiment E788 on Λ4He, is interpreted within the IPSM. We found that the shapes of all the spectra are basically tailored by the kinematics of the corresponding phase space, depending very weakly on the dynamics, which is gauged here by the one-meson-exchange-potential. In spite of the straightforwardness of the approach a good agreement with data is achieved. This might be an indication that the final-state-interactions and the two-nucleon induced processes are not very important in the decay of this hypernucleus. We have also found that the π+K exchange potential with soft vertex-form-factor cutoffs (Λπ≅0.7 GeV, ΛK≅0.9 GeV), is able to account simultaneously for the available experimental data related to Γp and Γn for Λ4H, and Λ5He.