Two-pion-exchange in the non-mesonic weak decay of Λ-hypernuclei

The non-mesonic weak decay of Λ-hypernuclei is studied within a one-meson-exchange potential supplemented by a chirally motivated two-pion-exchange mechanism. The effects of final state interactions on the outgoing nucleons are also taken into account. Particular attention is payed to the asymmetry of the protons emitted by polarized hypernuclei. The one-meson-exchange model describes the non-mesonic rates and the neutron-to-proton ratio satisfactorily but predicts a too large and negative asymmetry parameter. The two-pion exchange mechanism modifies the strength and sign of some decay amplitudes. As a consequence, while the rates change moderately, the asymmetry parameter is strongly affected, acquiring values that lie well within the experimental observations.