Comprehensive Study of Neutrino-Dark Matter Mixed Damping
  Mixed damping is a physical effect that occurs when a heavy species is
coupled to a relativistic fluid which is itself free streaming. As a cross-case
between collisional damping and free-streaming, it is crucial in the context of
neutrino-dark matter interactions. In this work, we establish the parameter
space relevant for mixed damping, and we derive an analytical approximation for
the evolution of dark matter perturbations in the mixed damping regime to
illustrate the physical processes responsible for the suppression of
cosmological perturbations. Although extended Boltzmann codes implementing
neutrino-dark matter scattering terms automatically include mixed damping, this
effect has not been systematically studied. In order to obtain reliable
numerical results, it is mandatory to reconsider several aspects of
neutrino-dark matter interactions, such as the initial conditions, the
ultra-relativistic fluid approximation and high order multiple moments in the
neutrino distribution. Such a precise treatment ensures the correct assessment
of the relevance of mixed damping in neutrino-dark matter interactions.
