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Small-scale structure formation properties of chilled sterile neutrinos as dark matter

Kalliopi Petraki

TL;DR

This work investigates sterile neutrino dark matter produced via decays of a gauge-singlet Higgs at the electroweak scale and its impact on small-scale structure. It develops the formalism for the free-streaming length $\lambda_{fs}$ and phase-space density $Q$ in terms of the production spectrum $n(x)$ and analyzes two production regimes: in-equilibrium and out-of-equilibrium decays. The results show that chilled sterile neutrinos from singlet-Higgs decays have a smaller $\lambda_{fs}$ and larger primordial $Q$ than Dodelson-Widrow production, due to their nonthermal spectra and entropy dilution, allowing them to constitute all dark matter within current observational bounds. The scenario links collider probes of the Higgs-singlet sector with cosmological small-scale structure data, offering testable predictions for keV-scale sterile-neutrino dark matter.

Abstract

We calculate the free-streaming length and the phase space density of dark-matter sterile neutrinos produced from decays, at the electroweak scale, of a gauge singlet in the Higgs sector. These quantities, which depend on the dark-matter production mechanism, are relevant to the study of small-scale structure formation and may be used to constrain or rule out dark-matter candidates.

Small-scale structure formation properties of chilled sterile neutrinos as dark matter

TL;DR

This work investigates sterile neutrino dark matter produced via decays of a gauge-singlet Higgs at the electroweak scale and its impact on small-scale structure. It develops the formalism for the free-streaming length and phase-space density in terms of the production spectrum and analyzes two production regimes: in-equilibrium and out-of-equilibrium decays. The results show that chilled sterile neutrinos from singlet-Higgs decays have a smaller and larger primordial than Dodelson-Widrow production, due to their nonthermal spectra and entropy dilution, allowing them to constitute all dark matter within current observational bounds. The scenario links collider probes of the Higgs-singlet sector with cosmological small-scale structure data, offering testable predictions for keV-scale sterile-neutrino dark matter.

Abstract

We calculate the free-streaming length and the phase space density of dark-matter sterile neutrinos produced from decays, at the electroweak scale, of a gauge singlet in the Higgs sector. These quantities, which depend on the dark-matter production mechanism, are relevant to the study of small-scale structure formation and may be used to constrain or rule out dark-matter candidates.

Paper Structure

This paper contains 6 sections, 12 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Free-Streaming and Phase Space Density
  3. Sterile Neutrinos produced from singlet Higgs decays
  4. Decays in equilibrium
  5. Out-of-equilibrium decays
  6. Conclusions

Figures (2)

  • Figure 1: The dimensionless time parameter $r=\frac{m_S}{T}$, corresponding to the peak of the sterile neutrino production rate from out-of-equilibrium $S$ boson decays, vs $\Lambda$. For the range of $\Lambda$ considered, $S$ boson decays peak at temperatures only a factor of a few below its mass, that is before the decoupling of the QCD degrees of freedom.
  • Figure 2: The free-streaming length and the phase space density for sterile neutrinos produced from out-of-equilibrium $S$ boson decays. $\lambda^{^{{\not} \Theta}}_{\rm fs}$ is in units of $\left(\frac{0.2}{\Omega}\right)^{\frac{1}{2}} \left(\frac{\: {\rm keV}}{m}\right) \; {\rm Mpc}$ and $Q^{^{{\not} \Theta}}$ is in units of $\left(\frac{\Omega}{0.2}\right) \left(\frac{m_s}{\: {\rm keV}}\right)^3 \; \frac{M_{_{\bigodot}}/{\rm pc}^3}{\rm (km/s)^3}$. Small $\Lambda$ implies delayed decays, resulting in warmer dark matter. Sterile neutrinos produced through this mechanism constitute all of dark matter for $\Lambda \approx 0.1$.