[Paper Review] How to find sterile neutrinos ?

Mikhail Shaposhnikov|ArXiv.org|Jun 13, 2007

Neutrino Physics Research2 references3 citations

TL;DR

This paper proposes the ν MSM, a minimal extension of the Standard Model with three light sterile Majorana neutrinos that simultaneously explains neutrino oscillations, dark matter, and baryon asymmetry. The model predicts keV–GeV scale sterile neutrinos with extremely weak couplings, making them detectable via X-ray decay signals or missing-energy signatures in high-intensity proton beams, with key constraints from cosmological and accelerator data.

ABSTRACT

We describe an extention of the Standard Model (the $ν$MSM) by three light singlet Majorana fermions -- sterile neutrinos, which allows to address simultaneously the problem of neutrino oscillations and the problems of dark matter and baryon asymmetry of the Universe. We discuss the ways these new particles can be searched for in astrophysical, laboratory, and accelerator experiments.

Motivation & Objective

• To unify explanations for neutrino oscillations, dark matter, and baryon asymmetry within a minimal extension of the Standard Model.
• To identify viable parameter space for light sterile neutrinos (1 keV–1 GeV) that satisfy cosmological and astrophysical constraints.
• To propose experimental strategies for detecting sterile neutrinos via X-ray spectroscopy and high-intensity proton beam experiments.
• To test the viability of the ν MSM against existing and future experimental results, including LHC, MiniBooNE, and dark matter searches.
• To rule out or confirm the model based on future observations such as WIMP detection, LSND confirmation, or neutrinoless double-beta decay.

Proposed method

• Extend the Standard Model with three right-handed sterile Majorana fermions (N_I), introducing 18 new parameters: 3 Majorana masses and 15 Yukawa couplings.
• Use the Lagrangian L_ν MSM = L_SM + L_N + L_Yukawa + L_Majorana to describe interactions, including Dirac and Majorana mass terms.
• Apply the see-saw mechanism with small Yukawa couplings (10^−6–10^−12) and keV–GeV scale Majorana masses to generate small active neutrino masses.
• Model dark matter as the lightest sterile neutrino (N_1) with a lifetime τ_N1 ∝ (M_1 / 1 keV)^−5 × (θ^2 / 10^−8)^−1, exceeding the age of the Universe for θ^2 ~ 10^−8.
• Propose accelerator-based searches using proton beams (e.g., NuMI, CNGS, J-PARC) to produce and detect sterile neutrinos via missing energy or decay signatures.
• Utilize space-based X-ray spectrometers with energy resolution δE/E ~ 10^−3–10^−4 to detect monoenergetic X-ray lines from N_1 decay (e.g., N_1 → νν̄ or ν̄ν).

Experimental results

Research questions

• RQ1 Can sterile neutrinos in the keV–GeV range simultaneously explain neutrino oscillations, dark matter, and baryon asymmetry within a minimal model?
• RQ2 What are the viable parameter ranges for sterile neutrino masses and mixing angles that satisfy cosmological and astrophysical constraints?
• RQ3 How can sterile neutrinos be detected in laboratory experiments using high-intensity proton beams and missing-energy signatures?
• RQ4 What role do X-ray observations of galactic and dwarf spheroidal sources play in detecting dark matter sterile neutrinos?
• RQ5 Which future experimental results would falsify the ν MSM, and how can the model be tested or ruled out?

Key findings

• The lightest sterile neutrino (N_1) can serve as a dark matter candidate with a lifetime exceeding the age of the Universe by ten orders of magnitude when M_1 ~ 1 keV and θ^2 ~ 10^−8.
• The decay width of N_1 is suppressed by small Yukawa couplings, leading to τ_N1 ~ 5×10^26 sec × (1 keV / M_1)^5 × (10^−8 / θ^2), enabling long-lived, weakly interacting dark matter.
• X-ray spectrometers with δE/E ~ 10^−3–10^−4 are required to detect the monoenergetic X-ray line from N_1 decay, particularly from the Milky Way and its dwarf satellites.
• Sterile neutrinos with masses below the kaon threshold (m_K ~ 490 MeV) can be searched for at existing accelerators like CNGS, FNAL, or J-PARC via missing energy in pion and kaon decays.
• For masses above the D-meson threshold (m_D ~ 1.86 GeV), detection becomes extremely challenging due to insufficient statistics and background, requiring beam intensity increases by two orders of magnitude.
• The ν MSM is falsifiable: discovery of WIMPs, confirmation of LSND anomaly, or observation of neutrinoless double-beta decay would rule out the model.