Weak decays of {sub {lambda}}{sup 4}He

We measured the lifetime and the mesonic and nonmesonic decay rates of the {sub {lambda}}{sup 4}He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K{sup -} beam on a liquid {sup 4}He target by the reaction {sup 4}He (K{sup -},{pi}{sup -}){sub {lambda}}{sup 4}He. The {sub {lambda}}{sup 4}He lifetime was directly measured using protons from {lambda}p{yields}np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of {tau}=245{+-}24 ps. The mesonic decay rates were determined from the observed numbers of {pi}{sup -}'s and {pi}{sup circle}'s as {gamma}{sub {pi}{sup -}}/{gamma}{sub tot}=0.270{+-}0.024 and {gamma}{sub {pi}}/{gamma}{sub tot}=0.564{+-}0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as {gamma}{sub p}/{gamma}{sub tot}=0.169{+-}0.019 and {gamma}{sub n}/{gamma}{sub tot}{<=}0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body {lambda}NN 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 {delta}I=(1/2) rule and the decay rates of the {sub {lambda}}{sup 4}H hypernucleus.