  In this letter we analyse the behavior of fidelity decay under a very
specific kind of perturbation: phase space displacements. Under these
perturbations, systems will decay following the Lyapunov regime only. Others
universal regimes discussed in the literature are not presented in this case;
instead, for small values of the perturbation we observe quantum freeze of the
fidelity. We also show that it is possible to connect this result with the
incoherent neutron scattering problem
