Weak decays of He-4(Lambda)

We measured the lifetime and the mesonic and nonmesonic decay rates of the (4)(Lambda) He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid He-4 target by the reaction He-4 (K-, pi(-))(Lambda)(4) He. The (4)(Lambda) He lifetime was directly measured using protons from Lambda p -> 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(-)'s and pi degrees's as Gamma(-)(pi)/Gamma(tot)=0.270 +/- 0.024 and Gamma(pi degrees)/Gamma(tot) = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma(p)/Gamma(tot) = 0.169 +/- 0.019 and Gamma(n)/Gamma(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 (4)(Lambda) H hypernucleus.