  We study the fidelity decay of the $k$-body embedded ensembles of random
matrices for bosons distributed over two single-particle states. Fidelity is
defined in terms of a reference Hamiltonian, which is a purely diagonal matrix
consisting of a fixed one-body term and includes the diagonal of the perturbing
$k$-body embedded ensemble matrix, and the perturbed Hamiltonian which includes
the residual off-diagonal elements of the $k$-body interaction. This choice
mimics the typical mean-field basis used in many calculations. We study
separately the cases $k=2$ and $3$. We compute the ensemble-averaged fidelity
decay as well as the fidelity of typical members with respect to an initial
random state. Average fidelity displays a revival at the Heisenberg time,
$t=t_H=1$, and a freeze in the fidelity decay, during which periodic revivals
of period $t_H$ are observed. We obtain the relevant scaling properties with
respect to the number of bosons and the strength of the perturbation. For
certain members of the ensemble, we find that the period of the revivals during
the freeze of fidelity occurs at fractional times of $t_H$. These fractional
periodic revivals are related to the dominance of specific $k$-body terms in
the perturbation.
