Chiral symmetry and meson exchange approach to hypernuclear decay
  We take an approach to the $\Lambda$ nonmesonic weak decay in nuclei based on the exchange of mesons under the guidelines of chiral Lagrangians. The one pion and one kaon exchange are considered, together with the exchange of two pions, either correlated, leading to an important scalar-isoscalar exchange ($\sigma$-like exchange), or uncorrelated (box diagrams). A drastic reduction of the OPE results for the $\Gamma_n/\Gamma_p$ ratio is obtained and the new results are compatible with all present experiments within errors. The absolute rates obtained for different nuclei are also in fair agreement with experiment. 
E. Oset, D. Jido and J.E. Palomar