Nucleon-nucleon coincidence spectra in the nonmesonic weak decay of Λ hypernuclei and the Γn/Γp puzzle -: art. no. 112501

By combining a one-meson-exchange model for the LambdaN-->nN transition in finite nuclei with an intranuclear cascade code, we have obtained nucleon-nucleon (angular and energy) coincidence distributions from the nonmesonic weak decay of He-5(Lambda) and C-12(Lambda) hypernuclei. Although, due to the elimination of interferences, two-nucleon coincidences are expected to give a cleaner extraction (with respect to single nucleon observables) of the ratio Gamma(n)/Gamma(p)equivalent toGamma(Lambdan-->nn)/Gamma(Lambdap-->np), we show that the effect of the final state interactions is still important even when applying favorable energy and angular cuts. The agreement of our results for the ratio N-nn/N-np between the number of nn and np emitted pairs with preliminary KEK coincidence data allows us to conclude that Gamma(n)/Gamma(p) for He-5(Lambda) should be small and close to the value of 0.46 predicted by our meson-exchange model.