A Theoretical Determination of Nnn/Nnp in Hypernuclear Non–Mesonic Weak Decay

The ratio Nnn/Nnp between the number of neutron–neutron and neutron–proton pairs emitted in the non–mesonic weak decay of Λ–hypernuclei is calculated within a nuclear matter formalism extended to via the local density approximation. The single–nucleon emission spectra, Np and Nn, are also evaluated. Our formalism takes care of both ground state correlations (gsc) and final state interactions (FSI). The evaluation of Nnn/Nnp — which, unlike Γn/Γp≡Γ(Λn→nn)/Γ(Λp→np), is an actual observable quantity in non–mesonic decay — is performed within a fully microscopic model where a proper treatment of FSI, gsc and ground state normalization is considered. All the isospin channels contributing to one– and two–nucleon induced decays are included. Our final result for the coincidence number ratio, Nnn/Nnp=0.374, is in agreement with the KEK–E508 datum, exp(Nnn/Nnp)=0.40±0.10.