Weak decays of Λ⁴He

We measured the lifetime and the mesonic and nonmesonic decay rates of the _Λ⁴He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K^- beam on a liquid ⁴He target by the reaction ⁴He (K^-,π^-)_Λ⁴He. The _Λ⁴He lifetime was directly measured using protons from Λp→np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of τ=245±24 ps. The mesonic decay rates were determined from the observed numbers of π^-'s and π^○'s as Γ_π^-/Γ_tot=0.270±0.024 and Γ_π^○/Γ_tot=0.564±0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Γ_p/Γ_tot=0.169±0.019 and Γ_n/Γ_tot⩽0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body ΛNN decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the nonmesonic branching fractions. The implications of the results of this analysis were considered for the empirical ΔI=1/2 rule and the decay rates of the _Λ⁴H hypernucleus.