Bound-state effects on dark matter coannihilation: Pushing the
  boundaries of conversion-driven freeze-out
  Bound-state formation can have a large impact on the dynamics of dark matter
freeze-out in the early Universe, in particular for colored coannihilators. We
present a general formalism to include an arbitrary number of excited bound
states in terms of an effective annihilation cross section, taking bound-state
formation, decay and transitions into account, and derive analytic
approximations in the limiting cases of no or efficient transitions.
Furthermore, we provide explicit expressions for radiative bound-state
formation rates for states with arbitrary principal and angular quantum numbers
$n,\ell$ for a mediator in the fundamental representation of $SU(3)_c$, as well
as electromagnetic transition rates among them in the Coulomb approximation. We
then assess the impact of bound states within a model with Majorana dark matter
and a colored scalar $t$-channel mediator. We consider the regime of
coannihilation as well as conversion-driven freeze-out (or coscattering), where
the relic abundance is set by the freeze-out of conversion processes. We find
that the region in parameter space where the latter occurs is considerably
enhanced into the multi-TeV regime. For conversion-driven freeze-out, dark
matter is very weakly coupled, evading direct and indirect detection
constraints but leading to prominent signatures of long-lived particles that
provide great prospects to be probed by dedicated searches at the upcoming LHC
runs.
