Dark matter bound-state formation at higher order: a non-equilibrium quantum field theory approach

Abstract

The formation of meta-stable dark matter bound states in coannihilating scenarios could efficiently occur through the scattering with a variety of Standard Model bath particles, where light bosons during the electroweak cross over or even massless photons and gluons are exchanged in the t-channel. The amplitudes for those higher-order processes, however, are divergent in the collinear direction of the in- and out-going bath particles if the mediator is massless. To address the issue of collinear divergences, we derive the bound-state formation collision term in the framework of non-equilibrium quantum field theory. The main result is an expression for a more general cross section, which allows to compute higher-order bound-state formation processes inside the primordial plasma background in a comprehensive manner. Based on this result, we show that next-to-leading order contributions, including the bath-particle scattering, are i) collinear finite and ii) generically dominate over the on-shell emission for temperatures larger than the absolute value of the binding energy. Based on a simplified model, we demonstrate that the impact of these new effects on the thermal relic abundance is significant enough to make it worthwhile to study more realistic coannihilation scenarios.

Coupled Boltzmann transport equations of heavy quarks and quarkonia in quark-gluon plasma

“…An effective field theory of QCD, potential nonrelativistic QCD (pNRQCD) [37][38][39] has been applied to study both the static screening of the potential and the dissociation rate [40][41][42]. Similar effective theory has also been used to study dark matter bound state formation [43][44][45]. Some studies also included viscous effects [46][47][48] and modifications due to the quarkonium moving with respect to a static medium [49,50].…”

Bound states of WIMP dark matter in Higgs-portal models. Part I. Cross-sections and transition rates

“…The dissipation of energy necessary for the capture into bound states or transitions between bound levels, may occur via exchange of an off-shell mediator with particles of the thermal bath [52][53][54][55][56][57][58]. 5 References [57,58] showed, in the context of a U(1) model, that the crosssections for BSF via scattering factorise into the radiative ones (with any phase-space suppression due to the mass of the emitted vector removed), and a factor that depends on the thermal bath and the interaction that mediates the scattering.…”

“…We also adapt the results of refs. [57,58] to our model, for BSF and transitions via off-shell B and W exchange.…”

Indirect searches for dark matter bound state formation and level transitions

“…Going from Eq. (30) to (31) assumes that either the velocity distribution of the DM particles remains the same along the line of sight, or that (σv rel ) BSF is velocity independent. In the present case, none of these assumptions is generally true, since (σv rel ) BSF is velocity dependent as discussed in Section 2, and the DM velocity distribution within the halos varies along with ρ DM .…”

“…In models that feature co-annihilation between different species and/or non-Abelian forces, a variety of unstable bound states may exist. The formation and decay of unstable bound states diminish the DM density in the early Universe [4], thereby altering the expected DM mass and couplings and affecting all experimental signatures [4,[22][23][24][25][26][27][28][29][30].…”