Dark Radiation Alleviates Problems with Dark Matter Halos

TL;DR

It is shown that a scalar and a fermion charged under a global U(1) symmetry can not only explain the existence and abundance of dark matter and dark radiation, but can also imbue DM with improved scattering properties at galactic scales, while remaining consistent with all other observations.

Abstract

We show that a scalar and a fermion charged under a global U(1) symmetry can not only explain the existence and abundance of dark matter (DM) and dark radiation (DR), but also imbue DM with improved scattering properties at galactic scales, while remaining consistent with all other observations. Delayed DM-DR kinetic decoupling eases the missing satellites problem, while DR mediated self-interactions of DM ease the cusp vs. core and too big to fail problems. In this scenario, DM is expected to be pseudo-Dirac and have a mass between 100 keV and 10 GeV. The predicted DR may be measurable using the primordial elemental abundances from big bang nucleosynthesis (BBN), and using the cosmic microwave background (CMB).

Figures (4)

• Figure 1: DM-DR scattering via $u$, $s$, and $t$ channels.
• Figure 2: Solution of all small-scale problems of DM. For a DM mass $m_\chi$, the coupling $\alpha_{\rm d}$ (top x-axis) is determined by the relic density. Small-scale problems are solved within the band. Three thin solid lines in the band correspond to $\langle\sigma_T\rangle/m_\chi \sim$ 0.1, 1, 10 $\text{cm}^2/\text{g}$, respectively (top-down), at $v_{\rm rel}\sim10\,{\rm km/s}$, addressing the cusp vs. core and too big to fail problems. The color-gradient inside shows the common logarithm of $\delta\equiv\Delta m_\chi/m_\chi$, which leads to $T_{\rm kd}=0.5\,{\rm keV}$ and solves the missing satellites problem. The hatched region at the bottom shows the constraint from galaxy clusters, $\langle\sigma_T\rangle/m_\chi \lesssim 1\,{\rm cm^2/g}$ at $v_{\rm rel}\sim10^3\,{\rm km/s}$, while the dashed line indicates the largest $m_\rho$ for which the scalar potential for $\phi$ is perturbative.
• Figure 3: $\Delta N_\nu$ generated by dark radiation, depending on the DM mass $m_\chi$ and the temperature $T_\star$ at which the dark and visible sectors decoupled. The white area denotes $\Delta N_\nu \geq 1$.
• Figure 4: Experimental upper bounds on the scalar mixing of $\phi$ and Higgs. Values of $m_\rho$ and $f_{\rm d}$ have been chosen to solve the small-scale problems. The shaded region has been excluded by either DM direct detection (dashed red), invisible Higgs decay (dotted blue), or bounds on $\Delta N_\nu$ (solid gray).