Self-interacting Dark Radiation

Abstract

We consider a simple class of models where dark radiation has self-interactions and therefore does not free stream. Such dark radiation has no anisotropic stress (or viscosity), leaving a distinct signature on the CMB angular power spectrum. Specifically we study a possibility that hidden gauge bosons and/or chiral fermions account for the excess of the effective number of neutrino species. They have gauge interactions and remain light due to the unbroken hidden gauge symmetry, leading to ΔN_{\rm eff} \simeq 0.29 in some case.

Figures (2)

• Figure 1: Dependence of $T_{\rm dec}$ (left panel) and $(g_{\ast \nu}/g_{\ast{\rm dec}})^{4/3}$ (right panel) on $\Lambda_\phi$. The Higgs decay rate into hidden gauge bosons is proportional to $N_g/\Lambda^4_\phi$, and it is larger than 1 MeV for $N_g=1$ in the shaded region. The branching ratio of this mode is ${\rm Br}(h\to V^\prime V^\prime)=20,\,10,\,1\%$ on the vertical lines, from left to right, for the case with $N_g=1$ and $2m_\phi>m_h$.
• Figure 2: Dark radiation from the hidden sector. The red solid (dashed) line shows $\Delta N_{\rm eff}$ for the hidden U(1)$^\prime$ with (without) massless fermions, where the number of fermions is set to be $N_f = 5$. As shown in the text, $N_f$ is bounded below $N_f \geq 5$ to satisfy the anomaly cancellation conditions. The dot-dashed lines are for larger hidden gauge groups without massless fermions. The values of $\Lambda_\phi$ leading to ${\rm Br}(h\to V^\prime V^\prime)=20,\,10,\,1\%$ for $2m_\phi>m_h$ are shown by the filled circles on each line, from left to right, respectively.