Dark Matter with Topological Defects in the Inert Doublet Model
  We examine the production of dark matter by decaying topological defects in
the high mass region $m_{\mathrm{DM}} \gg m_W$ of the Inert Doublet Model,
extended with an extra U(1) gauge symmetry. The density of dark matter states
(the neutral Higgs states of the inert doublet) is determined by the interplay
of the freeze-out mechanism and the additional production of dark matter states
from the decays of topological defects, in this case cosmic strings. These
decays increase the predicted relic abundance compared to the standard
freeze-out only case, and as a consequence the viable parameter space of the
Inert Doublet Model can be widened substantially. In particular, for a given
dark matter annihilation rate lower dark matter masses become viable. We
investigate the allowed mass range taking into account constraints on the
energy injection rate from the diffuse $\gamma$-ray background and Big Bang
Nucleosynthesis, together with constraints on the dark matter properties coming
from direct and indirect detection limits. For the Inert Doublet Model
high-mass region, an inert Higgs mass as low as $\sim 200$ GeV is permitted.
There is also an upper limit on string mass per unit length, and hence the
symmetry breaking scale, from the relic abundance in this scenario. Depending
on assumptions made about the string decays, the limits are in the range
$10^{12}$ GeV to $10^{13}$ GeV.
