  We present a GUT-model-independent calculation of hadron matrix elements for
all dimension-six operators associated with baryon number violating processes
using lattice QCD. Our results cover all the matrix elements required to
estimate the partial lifetimes of (proton,neutron)$\to$($\pi,K,\eta$) +(${\bar
\nu},e^+,\mu^+$) decay modes. We point out the necessity of disentangling two
form factors that contribute to the matrix elements; previous calculations did
not make the separation, which led to an underestimate of the physical matrix
elements. With a correct separation, we find that the matrix elements have
values 3-5 times larger than the smallest estimates employed in
phenomenological analyses of the nucleon decays, which gives stronger
constraints on GUT models. We also find that the values of the matrix elements
are comparable with the tree-level predictions of chiral Lagrangian.
