Mesonic and nonmesonic weak decay widths of medium-heavy Λ hypernuclei

We have measured the energy spectra of pions and protons emitted in the weak decay of C, Si, and Fe hypernuclei produced via the (π⁺,K⁺) reaction. The decay widths of the π^- mesonic decay (Λ→pπ^-) and the nonmesonic decay (ΛN→NN) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron- to proton-induced nonmesonic decay widths, Γ_n(Λn→nn)/Γ_p(Λp→np), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the Γ_n/Γ_p ratios in the present data.