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 Γπ-/Γtot=0.270±0.024 and Γπ○/Γtot=0.564±0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Γp/Γtot=0.169±0.019 and Γn/Γtot⩽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.</p