Direct Quark Mechanism for Weak $\Lam N \leftrightarrow NN$ Processes

Two body weak processes $\Lambda N \leftrightarrow NN$ are studied from the viewpoint of quark substructure of baryons.They can be studied in nonmesonic weak decays of hypernuclei and also hyperon production in the $NN$ scattering. The direct quark mechanism in contrast to the one-pion exchange mechanism is shown to give sizable $\di=3/2$ contribution in the J=0 channel. Nonmesonic decay rates of $^4_{\Lambda}$H, $^4_{\Lambda}$He, and $^5_{\Lambda}$He agree with the current experimental data fairly well. Especially the $n/p$ ratio is enhanced by the direct quark contribution. The cross sections for $pn\to \Lam p$ scattering are studied in the quark cluster model approach.