Nucleon-Nucleon Coincidence Spectra in the Nonmesonic Weak Decay of Hypernuclei

112501-1 By combining a one-meson-exchange model for the N ! 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 5 He and 12 C 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 n= p n ! nn = p ! 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 Nnn=Nnp between the number of nn and np emitted pairs with preliminary KEK coincidence data allows us to conclude that n= p for 5 He should be small and close to the value of 0.46 predicted by our meson-exchange model.