We take an approach to the Λ non-mesonic weak decay in nuclei based on the exchange of mesons. 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 (σ-like exchange), or uncorrelated (box diagrams). Extra effects of omega exchange in the scalar-isoscalar channel are also considered. Constraints of chiral dynamics are used to generate these exchanges. A drastic reduction of the OPE results for the Γ n /Γ 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 good agreement with experiment.[Key Word] Λ weak decay, Γ n /Γ p ratio, chiral unitary theory.

We develop a unitarity approach to consider the final state interaction corrections to the tree level graphs calculated from chiral perturbation theory (PT) allowing the inclusion of explicit resonance fields. The method is discussed considering the coupled channel pion and kaon vector form factors. These form factors are then matched with the one loop PT results. A very good description of experimental data is accomplished for the vector form factors up to ͱsр1.2 GeV beyond which multiparticle states play a non-negligible role. For the P-wave phase shifts the agreement with data stands even higher up to ͱsр1.5 GeV. We also consider the isospin breaking effects due to the -mixing as a perturbation to the previous results. In addition the low and resonance energy regions are discussed in detail and for the former a comparison with one and two loop PT is made showing a remarkable coincidence with the two loop PT results.

We make a theoretical study of the J/Ψ decays into ωππ, φππ, ωKK and φKK using the techniques of the chiral unitary approach stressing the important role of the scalar resonances dynamically generated through the final state interaction of the two pseudoscalar mesons. We also discuss the importance of new mechanisms with intermediate exchange of vector and axial-vector mesons and the role played by the OZI rule in the J/Ψφππ vertex, quantifying its effects. The results nicely reproduce the experimental data for the invariant mass distributions in all the channels considered.

We classify A-nucleon forces according to their isospin dependence and discuss the most general isospin structure of the three-nucleon force. We derive the leading and subleading isospin-breaking corrections to the three-nucleon force using the framework of chiral effective field theory.

We study the radiative φ decay into π 0 π 0 γ and π 0 ηγ taking into account mechanisms in which there are two sequential vector-vector-pseudoscalar or axial-vectorvector-pseudoscalar steps followed by the coupling of a vector meson to the photon, considering the final state interaction of the two mesons. There are other mechanisms in which two kaons are produced through the same sequential mechanisms or from φ decay into two kaons and then undergo final state interaction leading to the final pair of pions or π 0 η, this latter mechanism being the leading one. The results of the parameter free theory, together with the theoretical uncertainties, are compared with the latest experimental results of KLOE at Frascati.