A Hidden Dark Matter Sector, Dark Radiation, and the CMB
  We consider theories where dark matter is composed of a thermal relic of weak
scale mass, whose couplings to the Standard Model (SM) are however too small to
give rise to the observed abundance. Instead, the abundance is set by
annihilation to light hidden sector states that carry no charges under the SM
gauge interactions. In such a scenario the constraints from direct and indirect
detection, and from collider searches for dark matter, can easily be satisfied.
The masses of such light hidden states can be protected by symmetry if they are
Nambu-Goldstone bosons, fermions, or gauge bosons. These states can then
contribute to the cosmic energy density as dark radiation, leading to
observable signals in the cosmic microwave background (CMB). Furthermore,
depending on whether or not the light hidden sector states self-interact, the
fraction of the total energy density that free-streams is either decreased or
increased, leading to characteristic effects on both the scalar and tensor
components of the CMB anisotropy that allows these two cases to be
distinguished. The magnitude of these signals depends on the number of light
degrees of freedom in the hidden sector, and on the temperature at which it
kinetically decouples from the SM. We consider a simple model that realizes
this scenario, based on a framework in which the SM and hidden sector are
initially in thermal equilibrium through the Higgs portal, and show that the
resulting signals are compatible with recent Planck results, while large enough
to be detected in upcoming experiments such as CMBPol and CMB Stage-IV.
Invisible decays of the Higgs into hidden sector states at colliders can offer
a complementary probe of this model.
