  Partial compositeness is a mechanism for the generation of fermion masses
which replaces a direct Higgs coupling to the fermions by a linear mixing with
heavy composite partners. We present the first calculation of the relevant
matrix element in a lattice model which is very close to a candidate theory
containing a composite Higgs boson and a partially composite top quark.
Specifically, our model is an SU(4) gauge theory coupled to dynamical fermions
in the fundamental and two-index antisymmetric (sextet) representations. The
matrix element we obtain is small and hence our result disfavors the scenario
of obtaining a realistic top mass in this model.
