Cosmology with self-interacting sterile neutrinos and dark matter - A pseudoscalar model

Abstract

Short baseline neutrino oscillation experiments have shown hints of the existence of additional sterile neutrinos in the eV mass range. Such sterile neutrinos are incompatible with cosmology because they suppress structure formation unless they can be prevented from thermalising in the early Universe or removed by subsequent decay or annihilation. Here we present a novel scenario in which both sterile neutrinos and dark matter are coupled to a new, light pseudoscalar. This can prevent thermalisation of sterile neutrinos and make dark matter sufficiently self-interacting to have an impact on galactic dynamics and possibly resolve some of the known problems with the standard cold dark matter scenario. Even more importantly it leads to a strongly self-interacting plasma of sterile neutrinos and pseudoscalars at late times and provides an excellent fit to CMB data. The usual cosmological neutrino mass problem is avoided by sterile neutrino annihilation to pseudoscalars. The preferred value of $H_0$ is substantially higher than in standard $Λ$CDM and in much better agreement with local measurements.

Figures (4)

• Figure 1: The contribution of the sterile neutrino to the relativistic energy density $\delta N_{\textrm{eff}} = N_{\textrm{eff}} - 3$ as a function of the coupling parameter $g_s$.
• Figure 2: 1D marginalised posteriors for $N_{\textrm{eff}}$ ( Top panel) and $H_0$ ( Bottom panel) obtained by assuming the pseudoscalar scenario and using only CMB data (black/solid line) and CMB data plus the $H_0$ prior (red/dotted line). ( Top panel) The green dash-dot line refers to the $\Lambda$CDM model ($N_{\textrm{eff}}=3.046$) and the purple line is the complete thermalization case ($N_{\textrm{eff}} \simeq 4$). ( Bottom panel) The green and the blue dash-dot lines show the posteriors obtained in the $\Lambda$CDM model using Planck and Planck+$H_0$, respectively. The $H_0$ prior is marked by the grey shaded region Riess:2011yx.
• Figure 3: Sommerfeld enhancement factor for $\ell=0$ due the potential in Eq. \ref{['eq:DMpotential']} for two extreme values of the ratio $(m_\chi/\Lambda_\text{BSM})$. Top panel:$(m_\chi/\Lambda_\text{BSM})=1.0$. Bottom panel:$(m_\chi/\Lambda_\text{BSM})=10^{-5}$. As discussed in the text, the dependence on the ratio $(m_\phi/m_\chi)$ is negligible.
• Figure 4: Constraints in $m_\chi-g_d$ space. The green region is ruled out from Eq. (\ref{['eq:thermalise']}) due to overproduction of $\phi$-particles from $\chi$-annihilations, while the purple region will have no effect on galactic dynamics, cf. Eq. (\ref{['eq:hard']}).