Perturbative Resonance during Dark Matter Production and Its
  Cosmological Implications
  To trace the unknown particle nature of dark matter (DM) from observations in
CMB, we study the co-evolution of DM density perturbation and primordial metric
perturbation in the early Universe. By analytically solving the improved
Einstein-Boltzmann equation to include all perturbative contributions from the
DM productions, we observe an amplified perturbative resonance that takes place
between DM density perturbation and scalar modes of metric perturbation. As
this perturbative resonance does not affect the tensor modes, it results in a
suppression of the tensor-to-scalar ratio of metric perturbation $r$. Then by
using a typical reheating model to specify cosmic background, we establish an
explicit relation between DM particle mass $m_\chi$ and $r$. As $r$ will be
further constrained in the coming round of primordial gravitational wave
detections, $m_\chi$ would be constrained accordingly.
