Dark matter kinetic decoupling with a light particle
  We argue that the acoustic damping of the matter power spectrum is not a
generic feature of the kinetic decoupling of dark matter, but even the
enhancement can be realized depending on the nature of the kinetic decoupling
when compared to that in the standard cold dark matter model. We consider a
model that exhibits a ${\it sudden}$ kinetic decoupling and investigate
cosmological perturbations in the ${\it standard}$ cosmological background
numerically in the model. We also give an analytic discussion in a simplified
setup. Our results indicate that the nature of the kinetic decoupling could
have a great impact on small scale density perturbations.
