Abstract
Light vector mediators can naturally induce velocity-dependent dark matter self-interactions while at the same time allowing for the correct dark matter relic abundance via thermal freeze-out. If these mediators subsequently decay into Standard Model states such as electrons or photons however, this is robustly excluded by constraints from the Cosmic Microwave Background. We study to what extent this conclusion can be circumvented if the vector mediator is stable and hence contributes to the dark matter density while annihilating into lighter degrees of freedom. We find viable parts of parameter space which lead to the desired self-interaction cross section of dark matter to address the small-scale problems of the collisionless cold dark matter paradigm while being compatible with bounds from the Cosmic Microwave Background and Big Bang Nucleosynthesis observations.
Highlights
In their simplest forms, these light mediator scenarios are under strong pressure from observations: a vector mediator ZD leads to s-wave annihilation and if it predominantly decays into SM states such as electrons or photons, the energy injection from late-time annihilations ψψ → ZDZD → SM generically violates the stringent bounds obtained from the CMB [28, 29]
The CMB constraints on energy injection during recombination as discussed in section 4.2 are illustrated in Fig. 4, where we show the parameter space spanned by the gauge coupling gD and the light DM mass mZD for different values of the mass of the heavy DM particle, mψ = 1, 10, 100 and 1000 GeV
The black dashed curves show contours of constant values of ΩZDh2/ΩDMh2, i.e. the fraction of DM composed of ZD. This fraction grows towards smaller values of gD, until at some point the cross section for ZDZD → hDhD [which scales as gD4, see eq (3.3)] gets so small that irrespective of the choice of the gauge coupling gψ controlling the relic density of ψ, the abundance of ZD alone overcloses the Universe
Summary
We extend the SM gauge group by a ‘dark’ gauge symmetry U (1)D, and introduce a vectorlike Dirac fermion ψ as well as a complex scalar σ charged under this new symmetry. Prior to symmetry breaking of the SM and dark gauge group, the Lagrangian of the model is given by. As pointed out recently in [37], the kinetic mixing term can be forbidden by imposing a dark charge conjugation symmetry, rendering ZD absolutely stable (as long as mZD < 2mψ). In contrast to C, nature is symmetric with respect to dark charge conjugation, the kinetic mixing operator FDμνBμν is forbidden Notice that this symmetry is still present after the spontaneous breaking of U (1)D via a vev of σ. The presence of the portal term proportional to λhD in the scalar potential leads to a mixing of HD and H; we denote the corresponding mass eigenstates by hD and h. The tree-level mass of ψ is not related to the breaking of U (1)D, and can be naturally at a different scale
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