Scale-invariant scalar field dark matter through the Higgs portal
  We discuss the dynamics and phenomenology of an oscillating scalar field
coupled to the Higgs boson that accounts for the dark matter in the Universe.
The model assumes an underlying scale invariance such that the scalar field
only acquires mass after the electroweak phase transition, behaving as dark
radiation before the latter takes place. While for a positive coupling to the
Higgs field the dark scalar is stable, for a negative coupling it acquires a
vacuum expectation value after the electroweak phase transition and may decay
into photon pairs, albeit with a mean lifetime much larger than the age of the
Universe. We explore possible astrophysical and laboratory signatures of such a
dark matter candidate in both cases, including annihilation and decay into
photons, Higgs decay, photon-dark scalar oscillations and induced oscillations
of fundamental constants. We find that dark matter within this scenario will be
generically difficult to detect in the near future, except for the promising
case of a 7 keV dark scalar decaying into photons, which naturally explains the
observed galactic and extra-galactic 3.5 keV X-ray line.
