A White Paper on keV Sterile Neutrino Dark Matter (1602.04816v2)

Abstract: We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved - cosmology, astrophysics, nuclear, and particle physics - in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

• The paper establishes that keV sterile neutrinos offer a compelling dark matter candidate by addressing limitations of active neutrinos and traditional WIMP models.
• It details production mechanisms—including non-resonant and resonant oscillations—and analyzes constraints from X-ray observations and the Lyman-alpha forest.
• The study integrates insights from cosmology, astrophysics, and particle physics to outline experimental search strategies and theoretical model frameworks.

This white paper provides a comprehensive review of keV-scale sterile neutrinos as a dark matter (DM) candidate, integrating perspectives from cosmology, astrophysics, nuclear physics, and particle physics, encompassing both theoretical and experimental/observational viewpoints (1602.04816).

The paper begins by establishing the context, reviewing the role of standard model (active) neutrinos and outlining the limitations of known particles as DM candidates. It highlights that while active neutrinos are too light and hot to constitute the observed DM, extensions beyond the Standard Model offer various candidates. The paper then contrasts the popular Weakly Interacting Massive Particle (WIMP) paradigm with the sterile neutrino hypothesis, noting the lack of conclusive WIMP detection and the emergence of discrepancies between purely Cold Dark Matter (CDM) simulations and observations on small scales (e.g., the missing satellite problem, the cusp-core problem, and the too-big-to-fail problem).

The core focus is on sterile neutrinos with masses in the keV range. This mass scale is motivated by theoretical considerations (like the Tremaine-Gunn bound derived from phase-space density limits for fermions) and observational constraints (primarily from the non-observation of X-ray decay lines, which sets an upper bound). The paper critically discusses the tentative observational evidence for a 3.5 keV X-ray line, which could potentially be interpreted as the decay signal of a ~7 keV sterile neutrino DM particle.

Several key aspects of keV sterile neutrino DM are explored in detail:

1. Production Mechanisms: The paper reviews various mechanisms by which keV sterile neutrinos could have been produced in the early Universe to account for the observed DM abundance. These include:
   • Thermal production via mixing: This involves oscillations between active and sterile neutrinos. Non-resonant production (Dodelson-Widrow mechanism) is the minimal scenario but often conflicts with X-ray bounds. Resonant production (Shi-Fuller mechanism), enhanced by a lepton asymmetry, can produce the required abundance with smaller mixing angles compatible with X-ray constraints and potentially explain the 3.5 keV line. This mechanism typically results in a "cooler" DM spectrum than non-resonant production.
   • Decays of heavier particles: Sterile neutrinos could be produced from the decay of other particles, such as scalars (including potentially the inflaton) or other heavy states existing in the early Universe. This can lead to different DM momentum distributions.
   • Thermal production with dilution: Sterile neutrinos might have reached thermal equilibrium early on if they participated in additional interactions (e.g., extended gauge symmetries), with their relic abundance subsequently reduced to observed levels by significant entropy production before Big Bang Nucleosynthesis (BBN).
2. Cosmological and Astrophysical Constraints: A detailed overview of constraints is provided, covering:
   • Phase-space density: Limits based on the maximum phase-space density allowed in dwarf spheroidal galaxies (Tremaine-Gunn bound and refinements).
   • Lyman-alpha forest: Constraints derived from the matter power spectrum probed by the absorption lines in quasar spectra, which are sensitive to the free-streaming properties (warmness) of DM. These bounds depend strongly on the assumed production mechanism and the resulting momentum distribution.
   • X-ray observations: Stringent limits from the search for mono-energetic photons from the decay $N \to \nu \gamma$. The paper discusses the status of the 3.5 keV line signal in detail.
   • Structure formation: Analysis of small-scale structure issues (missing satellites, core-cusp, too-big-to-fail) and how WDM scenarios, like keV sterile neutrinos, might alleviate these tensions compared to CDM.
   • BBN and CMB: Effects on light element abundances and cosmic microwave background anisotropies, primarily constraining the effective number of relativistic species ($N_{\rm eff}$) and the total neutrino mass sum.
3. Particle Physics Model Building: The paper reviews theoretical models that explain the origin of keV-scale sterile neutrino masses, often requiring mechanisms to generate a hierarchy between different sterile neutrino mass states. Examples include split seesaw, Froggatt-Nielsen mechanisms, models based on flavor symmetries ($L_e-L_\mu-L_\tau$, $Q_6$, $A_4$), radiative inverse seesaw, and composite neutrino models.
4. Experimental Searches: Current and future efforts to detect keV sterile neutrinos are discussed:
   • Astrophysical searches: Primarily X-ray telescopes (XMM-Newton, Chandra, Suzaku, NuSTAR, and future missions like Athena) searching for the decay line, and Lyman-alpha forest surveys. Supernova physics and pulsar kicks also offer potential indirect probes.
   • Laboratory searches: Direct detection experiments aiming to observe the relic sterile neutrinos are challenging but being explored. More accessible are searches for kinematic effects in nuclear decays, such as kinks in the $\beta$-spectra of tritium (KATRIN, Troitsk, Project 8, PTOLEMY) or electron capture spectra of isotopes like $^{163}$Ho (ECHo, HOLMES, NuMECS). The SHiP experiment aims to search for heavier (GeV-scale) sterile neutrinos predicted in frameworks like the $\nu$MSM, which could indirectly support the existence of a keV DM sterile neutrino.

The White Paper concludes by summarizing the status, highlighting the theoretical motivations, the observational hints and tensions, the various constraints, and the prospects for future discovery or exclusion, emphasizing the multi-disciplinary nature of the field.