Bound-state effects on dark matter coannihilation: Pushing the boundaries of conversion-driven freeze-out

Abstract

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 as well as 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, 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.

Impact of bound states on non-thermal dark matter production

“…The results from this prescription agree with those obtained following the method proposed in [17,33] to 1% throughout the parameter space considered here. This method can also be generalized to include transitions between bound states which are neglected here [34,35]. For illustration, we show the solutions to the Boltzmann equation for an exemplary parameter point with m t = 5 TeV and λ H = 0 in Fig.…”