Neutron-proton coincidences from Non Mesonic Weak Decay of p-shell Lambda-hypernuclei and determination of the two-nucleon induced process

The decay of Λ-hypernuclei without π emission, known as Non-Mesonic Weak Decay (NMWD), gives an effective tool to investigate ΔS=1ΔS=1 four-baryon interactions. It was theoretically suggested that the two-nucleon induced mechanism could play a substantial role in reproducing the observed NMWD decay rates and nucleon spectra, but at present no direct evidence of such a mechanism has been obtained. The FINUDA experiment, exploiting the possibility to detect both charged and neutral particles coming from the hypernucleus decay, has allowed us to deduce the relative weight of the two nucleon induced decay rate to the total NMWD rate. The value of View the Γ2N/ΓNMWD=0.21±0.07stat−0.02sys+0.03sys has been deduced, well consistent with the previous determinations.