Abstract
We present a simple model of dark matter that can address astrophysical and cosmological puzzles across a wide range of scales. The model is an application of the Secretly Asymmetric Dark Matter mechanism, where several flavors of dark matter are fully asymmetric despite an exact dark matter number symmetry $U(1)_{\chi}$. The dark matter relic abundance arises from these asymmetries, generated in the early universe through right-handed neutrino decays. The $U(1)_{\chi}$ is gauged by a massless dark photon, and asymmetries with opposite signs in the different DM flavors result in the formation of bound states. Dark acoustic oscillations in the early universe lead to a suppression in the matter power spectrum for addressing the $\sigma_8$ problem. The dark photon as a relativistic degree of freedom contributes to $\Delta N_{eff}$, easing the tension between the CMB and late-time $H_{0}$ measurements. Finally, scattering between the bound states after structure formation leads to a flattening of the dark matter distribution at the cores of haloes.
Highlights
Asymmetric dark matter (SADM) has recently been proposed [1] as a mechanism to generate the dark matter (DM) relic abundance through an asymmetry in the dark sector despite an exact DM number symmetry Uð1Þχ
If λ2 ∼ Y2 ∼ 0.1 and αd ∼ α, ε is below 10−10. Let us compare this to the constraints for millicharged DM [Fig. 1 in Ref. [43], with m ∼ Oð10Þ GeV, the mass of the heaviest DM flavor], keeping in mind that some of the constraints will be relaxed, as these were derived with the assumption that all DM particles are millicharged while we focus in this work exclusively on the possibility that all but a small fraction of the χ particles are in bound states, and have no net millicharge
As a final consideration about the particle physics nature of our model, we address the potential concern that flavor oscillations between the DM flavors may cause a washout of the asymmetries, as the particles of χ1 oscillate into the antiparticles of χ2 or χ3, or the other way around
Summary
Asymmetric dark matter (SADM) has recently been proposed [1] as a mechanism to generate the dark matter (DM) relic abundance through an asymmetry in the dark sector despite an exact (global or gauged) DM number symmetry Uð1Þχ. Where the heavier DM flavors decay to the lightest one before big bang nucleosynthesis (BBN) and result in a symmetric distribution of the lightest DM flavor at late times, in the model we study in this work all DM flavors are extremely long lived and are still present today This means that flavors with opposite signs of asymmetry (and with opposite dark charges) must coexist, along with a massless dark photon that mediates long-range interactions, and multiple bound states can form, each one behaving as atomic DM [21,38,39]. At dimension 5, this generates the operator [we use λ to stand in for any Oð1Þ entries in the λij coupling matrix, and we are suppressing flavor indices on the χ] Note that this operator does not contribute to the decay mode of Fig. 2, since one needs a factor of φφà in order to close the φ loop. The large mass gap between the χ flavors in the parameter region of interest in this paper, and the extremely small intrinsic widths of these states, makes oscillations so small that they are negligible for all intents and purposes
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