Sterile Neutrino Dark Matter as a Probe of Inflationary Reheating

Abstract

Sterile neutrinos offer a minimal and testable explanation for dark matter (DM), with their radiative decay actively searched for in X-ray observations. We show that cold sterile neutrino DM can be efficiently produced during reheating from inflaton decays with a tiny branching ratio, ${\rm BR}\lesssim 10^{-4}$. This production mechanism opens regions of parameter space where the active-sterile mixing is small enough to evade current X-ray constraints while reproducing the observed DM abundance. We systematically map the viable parameter space in terms of the sterile neutrino mass, mixing angle, inflaton mass, reheating temperature, and branching ratio. We further demonstrate that sterile neutrino DM can serve as a probe of inflationary reheating, with future X-ray observations capable of setting lower bounds on the reheating temperature several orders of magnitude above the existing bound from Big Bang Nucleosynthesis.

Figures (3)

• Figure 1: Branching ratio for inflaton decay to sterile neutrinos (BR) as a function of $m_\phi/T_\text{rh}$, required to reproduce the observed DM abundance.
• Figure 2: Sterile neutrino DM parameter space $\{\sin^2(2 \theta), m_s\}$ for exemplary choices of BR and $T_\text{rh}$, assuming fixed inflaton mass $m_\phi = 1$ GeV. Stars exemplify regions where BDKDW production is significant; see text for details.
• Figure 3: Evolution of the neutrino spectra $x^{3} f(x)$ with decreasing temperature. Rows correspond to different admixtures of oscillations and decays, as described in the text.