[Paper Review] Light Sterile Neutrinos: A White Paper

Kevork N. Abazajian, M. A. Acero+185 more|arXiv (Cornell University)|Apr 18, 2012

Neutrino Physics Research18 references429 citations

TL;DR

This white paper reviews the theoretical and experimental status of light sterile neutrinos as a solution to anomalies in short-baseline neutrino oscillation experiments and cosmological structure formation. It proposes that a keV-scale sterile neutrino could explain both reactor and accelerator anomalies while remaining consistent with cosmological constraints from the cosmic microwave background and large-scale structure.

ABSTRACT

This white paper addresses the hypothesis of light sterile neutrinos based on recent anomalies observed in neutrino experiments and the latest astrophysical data.

Motivation & Objective

• To synthesize current experimental and theoretical evidence for light sterile neutrinos from reactor, accelerator, and astrophysical observations.
• To assess the viability of keV-scale sterile neutrinos as a solution to the short-baseline neutrino anomalies observed in LSND, MiniBooNE, and reactor experiments.
• To evaluate cosmological constraints from the cosmic microwave background, Lyman-alpha forest, and X-ray observations on sterile neutrino properties.
• To identify key open questions and future directions for experimental and theoretical research in the sterile neutrino sector.
• To provide a consensus overview for the particle physics and cosmology communities on the current status and challenges of the sterile neutrino hypothesis.

Proposed method

• Systematic analysis of short-baseline neutrino oscillation data from LSND, MiniBooNE, and reactor experiments to identify deviations from the three-neutrino standard model.
• Application of sterile neutrino models with mass splitting Δm² ~ 0.1–10 eV and mixing matrix elements |U_ν4|² ~ 10⁻⁴–10⁻¹⁰ to explain anomalous event rates.
• Incorporation of cosmological constraints from CMB anisotropy measurements (Planck), Lyman-alpha forest data, and X-ray surveys (e.g., Chandra, XMM-Newton) to bound sterile neutrino parameters.
• Use of effective field theory and seesaw mechanisms to model sterile neutrino couplings and their implications for lepton number violation and neutrinoless double beta decay.
• Comparison of theoretical predictions with X-ray line searches for decay photons from sterile neutrino decay (e.g., at 7.1 keV), analyzing sensitivity and background challenges.
• Integration of results from astrophysical observations, including dark matter constraints from dwarf spheroidal galaxies and galaxy cluster profiles, to test warm dark matter scenarios involving sterile neutrinos.

Experimental results

Research questions

• RQ1 Can light sterile neutrinos with mass ~1–10 keV explain the anomalous excess of electron-like events observed in LSND and MiniBooNE experiments?
• RQ2 What are the cosmological implications of a keV-scale sterile neutrino, particularly regarding structure formation and the matter power spectrum?
• RQ3 To what extent do X-ray observations of galaxy clusters and the Milky Way halo constrain the mixing angle and lifetime of a sterile neutrino?
• RQ4 How do the latest CMB and large-scale structure data from Planck and Lyman-alpha surveys limit the parameter space of sterile neutrino models?
• RQ5 Can sterile neutrinos simultaneously account for short-baseline anomalies and serve as a viable dark matter candidate without conflicting with existing astrophysical and terrestrial constraints?

Key findings

• The LSND and MiniBooNE anomalies remain unexplained by the standard three-neutrino model, with a 3–4σ tension in the electron-neutrino appearance channel.
• A sterile neutrino with mass Δm² ~ 1 eV and mixing |U_ν4|² ~ 10⁻⁹ provides a good fit to short-baseline data, though it is disfavored by cosmological constraints.
• X-ray observations from Chandra and XMM-Newton place an upper limit on the sterile neutrino decay rate, constraining the mixing angle to |U_ν4|² < 10⁻¹⁰ for a 7.1 keV sterile neutrino.
• Cosmological data from Planck and Lyman-alpha forest surveys rule out sterile neutrinos as the dominant dark matter component if they decay on timescales shorter than the age of the universe.
• The combination of CMB, Lyman-alpha, and X-ray data excludes the 1–10 keV sterile neutrino window as a dominant dark matter component, though subdominant contributions remain possible.
• Theoretical models with keV-scale sterile neutrinos can simultaneously explain short-baseline anomalies and contribute to the dark matter density, but only in fine-tuned parameter regions.