Nonmesonic Weak Decay of $\Lambda$-Hypernuclei within Independent-Particle

F. KRMPOTIC, A. P. GALEÃO, AND M. S. HUSSEIN

Aguas de Lindoa, Sao Paulo, Brasil

Workshop; XXXII BRAZILIAN WORKSHOP ON NUCLEAR PHYSICS; 2009

Sociededade Brasileira de F¨ªsica

After a short introduction to  the nonmesonic weak decay (NMWD)$Lambda N--> nN$ of $Lambda$-hypernuclei we discuss thelong-standing puzzle on the ratio $Gamma_n/Gamma_p$, and somerecent  experimental evidences that signalized towards  its finalsolution. Two versions of the  Independent-Particle-Shell-Model(IPSM) are employed to account for the nuclear structure of  thefinal residual nuclei. They are: (a) IPSM-a, where  nocorrelation, except for the Pauli principle, is  taken intoaccount, and (b) IPSM-b, where the highly excited  hole states areconsidered to be quasi-stationary and are described byBreit-Wigner distributions, whose widths are estimated from theexperimental data. We evaluate  the  coincidence spectra in$^{4}_Lambda$He, $^{5}_Lambda$He, $^{12}_Lambda$C,$^{16}_Lambda$O, and $^{28}_Lambda$Si, as a function of the  sumof kinetic energies $E_{nN}=E_n+E_N$ for $N=n,p$.  The recentBrookhaven National Laboratory experiment E788 on $^4_Lambda$He,is interpreted within the IPSM . We found that  the shapes of allthe spectra are basically tailored by the kinematics of thecorresponding phase space, depending very weakly on the dynamics,which is gauged here by the one-meson-exchange-potential. In spiteof the straightforwardness of the approach a good agreement withdata is achieved. This might be an indication that thefinal-state-interactions and the two-nucleon induced processes arenot very important  in the decay of this hypernucleus. We havealso found that the $pi+K$ exchange potential with softvertex-form-factor cutoffs $(Lambda_piapprox 0.7$ GeV,$Lambda_Kapprox 0.9$ GeV), is able to account simultaneously forthe available experimental data related to $Gamma_p$ and$Gamma_n$ for  $^4_Lambda$H, $^4_Lambda$He, and $^5_Lambda$He.