| The study of the decay of hypernuclei, or nuclei which contain one or more hyperons, can provide insight into the nature of the YN weak interaction. In particular, the reaction ΛN → nN, referred to as nucleon-stimulated Λ decay, is useful for probing the isospin structure of the Λ-nucleon weak interaction and can be most easily studied in the hypernuclear environment. For free Λ decay, the change in total isospin, DeltaI, is governed by the empirical DeltaI = ½ rule. This rule is a statement of the observed dominance of DeltaI = ½ transitions over DeltaI = 32 transitions for most strangeness-changing weak decays. A measurement of the proton-stimulated and neutron-stimulated decay rates for several hypernuclear species provides a means of testing the DeltaI = ½ rule for nucleon-stimulated decay modes.; Brookhaven National Laboratory experiment E788 was designed to measure the decay rates of the H4Le hypernucleus. The hypernuclei are created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction K- + 4He → pi - + H4Le . The bound Λ particle then decays via the mesonic decay modes, Λ → ppi- and Λ → npi°, or the non-mesonic nucleon-stimulated decay modes, Λp → pn and Λn → nn, with the associated decay rates Gammapi-, Gamma pi°, Gammap, and Gamma n, respectively. The mesonic decay rates are determined from the observed numbers of pi-'s and pi°'s as Gamma pi-/Gammatot = 0.269 +/- 0.026 and Gammapi°/Gamma tot = 0.552 +/- 0.098, respectively. The non-mesonic decay rates are extracted from the observed single particle proton and neutron kinetic energy spectra using a model which incorporates final-state interactions and 3-body ΛNN decay contributions. Also, a study of events with two detected nucleons (pn, nn, and pp coincidences) provides another means of extracting the non-mesonic decay rates and produces results consistent with the single particle spectra. The values of the proton- and neutron-stimulated decay rates are found as Gamma p/Gammatot = 0.161 +/- 0.015 and Gamman/Gammatot ≤ 0.013, respectively (as determined by a weighted average of the results of the two determinations). Finally, the implications of the results of this analysis are considered for the DeltaI = ½ rule and the decay of the H4L hypernucleus. |
