Nonmesonic weak decays of s-shell hypernuclei are analyzed in microscopic models for the $\Lambda N\to NN$ weak interaction. A scalar-isoscalar meson σ is introduced, and its importance in accounting for the decay rates, $n/p$ ratios, and proton asymmetry is demonstrated. Possible violation of the $\Delta I=1/2$ rule in the nonmesonic weak decay of the Λ is discussed in a phenomenological analysis, and several useful constraints are presented. The microscopic calculation shows that the current experimental data indicate a large violation of the $\Delta I=1/2$ rule, although no definite conclusion can be derived because of the large ambiguity of the decay rate of ${}_{\Lambda }^{4}H$.

• Received 7 October 2004

DOI:https://doi.org/10.1103/PhysRevC.71.035502