  The nonmesonic weak decay of $\Lambda$ hypernuclei is studied in a shell
model framework. A complete strangeness-changing weak $\Lambda N \to NN$
transition potential, based on one boson exchange, is constructed by including
the exchange of the pseudoscalar mesons $\pi$, K, $\eta$ as well as the vector
mesons $\rho, \omega$, and K$^*$, whose weak coupling constants are obtained
from soft meson theorems and SU(6)$_w$. General expressions for nucleons in
arbitrary shells are obtained. The transition matrix elements include realistic
$\Lambda$N short-range correlations and NN final state interactions based on
the Nijmegen baryon-baryon potential. The decay rates are found to be
especially sensitive to the inclusion of the strange mesons, K and K$^*$, even
though the role of kaon exchange is found to be reduced with recent couplings
obtained from next-to-leading order Chiral Perturbation Theory. With the weak
couplings used in this study the rates remain dominated by the pion-exchange
mechanism since the contributions of heavier mesons either cancel each other or
are suppressed by form factors and short-range correlations. The total decay
rate therefore remains in agreement with present measurements. However, the
partial rates which are even more sensitive to the inclusion of heavier mesons
cannot be reconciled with the data. The proton asymmetry changes by 50% once
heavier mesons are included and agrees with the available data.
