NONMESONIC WEAK DECAYS OF LIGHT HYPERNUCLEI IN THE DIRECT QUARK AND THE ONE-PION EXCHANGE

Contributions of the direct quark mechanism are studied in the non-mes onic weak decays of light hypernuclei. The Lambda N --> NN transition is described by the one pion exchange mechanism and the direct quark m echanism, induced by the four-quark vertices in the effective weak Ham iltonian. By employing a realistic wave function of the Lambda inside the hypernuclei, non-mesonic decay rates of H-4(Lambda), He-4(Lambda), and He-5(Lambda) are calculated. The results show that the direct qua rk mechanism is significantly large and gives a large Delta I = 3/2 co ntribution in the J = 0 channel. The relative phase between the one-pi on exchange and the direct-quark contributions is determined so that t he effective weak Hamiltonian for quarks gives both of them consistent ly. We find that the sum of these two contributions reproduce the curr ent available experimental data fairly well. (C) 1998 Elsevier Science B.V.