  The weak $\Lambda N\to NN$ transition is studied in the valence quark model
approach. The quark component of the two baryon system is described in the
quark cluster model and the weak transition potential is calculated by
evaluating the matrix elements of the $\Delta S=1$ effective weak Hamiltonian.
The transition potential is applied to the decay of hypernuclei and the results
are compared with available experimental data. The results indicate that direct
quark process is significant and qualitatively different when compared with
those in conventional meson-exchange calculations. The direct quark mechanism
predicts the violation of the $\Delta I = 1/2$ rule for this transition.

High Energy Physics - Phenomenology