Sterile neutrinos as subdominant warm dark matter

TL;DR

This work addresses the viability of sterile neutrinos as warm dark matter produced by the non-resonant Dodelson-Widrow mechanism while relaxing the common assumption that they constitute all of the dark matter. By reinterpreting diffuse X-ray background and Ly-$\alpha$ forest data for a subdominant relic abundance $f_s$, the authors map experimental constraints onto the DW parameter space $(m_s, \sin^2 2\theta)$ as a function of $f_s$ and incorporate theoretical uncertainties in the production yields. The analysis yields a robust upper limit $f_s\lesssim 0.7$ (2$\sigma$), with $f_s=1$ strongly disfavored (~$3\sigma$), and identifies viable DW-produced regions at intermediate $f_s$ values (e.g., $m_s\sim$ a few keV up to $\sim16$ keV and $\sin^2 2\theta\sim10^{-11}$–$10^{-9}$) depending on $f_s$. These results demonstrate that DW-produced sterile neutrino dark matter can still be viable if subdominant, and they quantify how current data and theoretical uncertainties shape the allowed parameter space, guiding future X-ray/Ly-$\alpha$ probes and the exploration of alternative production mechanisms.

Abstract

In light of recent findings which seem to disfavor a scenario with (warm) dark matter entirely constituted of sterile neutrinos produced via the Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for this mechanism by relaxing the usual hypothesis that the relic neutrino abundance must necessarily account for all of the dark matter. We first study how to reinterpret the limits attainable from X-ray non-detection and Lyman-alpha forest measurements in the case that sterile neutrinos constitute only a fraction fs of the total amount of dark matter. Then, assuming that sterile neutrinos are generated in the early universe solely through the DW mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the mass-mixing parameters governing their production. Furthermore, we show how the same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level, rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.

Figures (6)

• Figure 1: Constraints (at the $3 \sigma$ level) on mass and mixing parameters obtained from the spectral analysis of the HEAO-1 data under the assumption that sterile neutrinos account for all the dark matter content ($\Omega_s = \Omega_{\mathrm {dm}}$). The bounds obtained including only the Milky Way (MW) contribution, only the extra-galactic (EG) contribution, and their sum are shown separately.
• Figure 2: Fractional suppression of the one dimensional matter power spectrum for mixed (CDM +WDM) models with respect to the case of pure CDM. The values refer to the pivot scale $k = 2h$ Mpc$^{-1}$. The region under the thick solid line is excluded at $2 \sigma$ level by Lyman-$\alpha$ measurements. See the text for details.
• Figure 3: Comparison of the constraints on mass-mixing parameters imposed by experimental data (X-ray and Ly-$\alpha$) with the theoretical predictions obtained in Asaka:2006rw for the Dodelson-Widrow mechanism. The two panels confront the "usual" case $f_s = 1$ (left panel) with a representative case in which sterile neutrinos constitute only a fraction $f_s = 0.2$ of dark matter (right panel).
• Figure 4: Joint constraints from X-ray and Ly-$\alpha$ data on the mass-mixing parameters obtained for three different theoretical estimates of the relic abundance of sterile neutrinos produced via the Dodelson-Widrow mechanism. In each panel the allowed region lies under the solid curve, the dotted lines represent curves of constant sterile neutrino fractional abundance, while the dashed segment indicates its maximum value allowed in each case. In the first panel the "average" theoretical calculation obtained in Asaka:2006rw is used. In the last two panels the theoretical abundance is taken equal to the two extreme estimates determined in Asaka:2006rw.
• Figure 5: Constraints on mass and mixing parameters governing the Dodelson-Widrow mechanism obtained by the joint analysis of X-ray and Ly-$\alpha$ data with the inclusion of the theoretical uncertainties. See the text for details.