We measured the lifetime and the mesonic and nonmesonic decay rates of the ${}_{\Lambda }^4\mathrm{He}$ hypernucleus. The hypernuclei were created using a 750 MeV/$c$ momentum $K^{-}$ beam on a liquid ${}^4\mathrm{He}$ target by the reaction ${}^4\mathrm{He}$ $(K^{-},{\pi }^{-}){}_{\Lambda }^4\mathrm{He}$. The ${}_{\Lambda }^4\mathrm{He}$ lifetime was directly measured using protons from $\Lambda p\to np$ nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of $\tau =245\pm 24$ ps. The mesonic decay rates were determined from the observed numbers of ${\pi }^{-}$'s and ${\pi }^{○}$'s as ${\Gamma }_{{\pi }^{-}}/{\Gamma }_{\mathrm{tot}}=0.270\pm 0.024$ and ${\Gamma }_{{\pi }^{○}}/{\Gamma }_{\mathrm{tot}}=0.564\pm 0.036$, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as ${\Gamma }_p/{\Gamma }_{\mathrm{tot}}=0.169\pm 0.019$ and ${\Gamma }_n/{\Gamma }_{\mathrm{tot}}⩽0.032$ (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 $\Delta I=\frac{1}{2}$ rule and the decay rates of the ${}_{\Lambda }^4\mathrm{H}$ hypernucleus.

• Received 21 May 2007
• Corrected 12 September 2007

DOI:https://doi.org/10.1103/PhysRevC.76.035501