The role of sterile neutrinos in cosmology and astrophysics

TL;DR

The paper presents the νMSM, a minimal Standard Model extension that adds three sterile neutrinos with sub-electroweak masses to simultaneously address neutrino oscillations, baryogenesis, and dark matter. It analyzes a concrete parameter set where $N_1$ yields warm or cold DM while $N_2,N_3$ drive CP-violating baryogenesis and decay before BBN, with inflation realizable via Higgs non-minimal coupling. The authors derive production mechanisms for DM (NRP and RP), explore structure-formation implications, and compile multifaceted constraints from X-ray searches, phase-space limits, and Lyman-$\alpha$ data, demonstrating that a consistent, testable region of parameter space exists. They also discuss experimental prospects for confirming or falsifying the model through astrophysical observations and high-intensity laboratory experiments, highlighting the model’s predictive power and falsifiability. Overall, the νMSM offers a cohesive, low-energy-scale framework connecting particle physics with cosmology and astrophysics, with clear avenues for verification.

Abstract

We present a comprehensive overview of an extension of the Standard Model that contains three right-handed (sterile) neutrinos with masses below the electroweak scale [the Neutrino Minimal Standard Model, (nuMSM)]. We consider the history of the Universe from the inflationary era through today and demonstrate that most of the observed phenomena beyond the Standard Model can be explained within the framework of this model. We review the mechanism of baryon asymmetry of the Universe in the nuMSM and discuss a dark matter candidate that can be warm or cold and satisfies all existing constraints. From the viewpoint of particle physics the model provides an explanation for neutrino flavor oscillations. Verification of the nuMSM is possible with existing experimental techniques.

Figures (4)

• Figure 1: Constraints on the masses and mixing angles of the dark matter sterile neutrino $N_1$ (a) and of two heavier sterile neutrinos $N_{2,3}$ (b). These constraints come from astrophysics, cosmology, and neutrino oscillation experiments. Abbreviations: BAU, baryon asymmetry of the Universe; BBN, big bang nucleosynthesis.
• Figure 2: (a) Characteristic form of the resonantly produce (RP) sterile neutrino spectra for $M_1 =3\;\mathrm{keV}$ and $L_6=16$ (red solid line) and its resonant and nonresonant components. Also shown are spectra for $L_6=10$ and $L_6=25$, all of which have the same form for $q\gtrsim 3$. All spectra are normalized to give the same DM abundance. For the spectrum with $L_6=16$, its nonresonantly produce (NRP) component ${f_\textsc{nrp}}\xspace \simeq 0.12$ ((red dashed-dotted line) and its colder, RP component (red dashed line) are shown separately. The spectrum with $L_6=10$ has ${f_\textsc{nrp}}\xspace \simeq 0.53$ and $L_6=25$ has ${f_\textsc{nrp}}\xspace \simeq 0.60$. The RP components happen to peak around $q_\text{res}\sim 0.25-1$. Their width and height depends on $L_6$ and $M_1$. (b) Dependence of the ratio of the average momentum of sterile neutrinos to that of the active neutrinos on mass $M_1$ and lepton asymmetry. The black solid curve represents the ratio of average momenta, computed over the exact NRP spectra to the ratio using the approximate analytic form shown in Eq. (\ref{['eq:19']}). The black solid curve is the ratio of average momenta, computed over exact NRP spectra to the ratio, computed using the approximate analytic form (\ref{['eq:19']}). For each mass there is lepton asymmetry for which the $\langle q\rangle$ reaches its minimum $\langle q\rangle_{min} \sim 0.3\langle q\rangle_{\nu_\alpha}$. The corresponding spectra are the most distinct from the NRP spectra. The fraction ${f_\textsc{nrp}}\xspace$ for the former can be $\lesssim 20\%$. Panel b reproduced from Ref. Laine:08a with permission.
• Figure 3: Transfer functions (TFs) for the nonresonant production (NRP) spectra where $M_1=14\;\mathrm{keV}$ and $M_1=3\;\mathrm{keV}$. Also shown is a resonant production (RP) spectrum where $M_1=3\;\mathrm{keV}$ and $L_6=16$ together with a cold/warm DM spectrum for $M = 3\;\mathrm{keV}$ and $F_\textsc{wdm}\xspace \simeq 0.2$ (blue dashed-dotted line). Vertical lines mark the free-streaming horizon of the nonresonant ($k^0_\textsc{fsh}$, left line) and resonant ($k_\textsc{fsh}^\text{res}$, right line) components. Abbreviations: CWDM, cold-plus-warm dark matter; WDM, warm dark matter.
• Figure 4: The allowed region of parameters for DM sterile neutrinos produced via mixing with the active neutrinos (unshaded region). The two thick black lines bounding this region represent production curves for nonresonant production (NRP) (upper line, $L_6=0$) and for resonant production (RP) (lower line, $L_6^{max}=700$) with the maximal lepton asymmetry, attainable in the $\nu$MSM Laine:08aShaposhnikov:08a. The thin colored curves between these lines represent production curves for (from top to bottom) $L_6 = 8,12,16,25,$ and $70$. The red shaded region in the upper right corner represents X-ray constraints Boyarsky:06cBoyarsky:06dBoyarsky:07aBoyarsky:07bLoewenstein:08 (rescaled by a factor of two to account for possible systematic uncertainties in the determination of DM content Boyarsky:06eBoyarsky:07a). The black dashed-dotted line approximately shows the RP models with minimal $\langle q\rangle$ for each mass, i.e., the family of models with the largest cold component. The black filled circles along this line are compatible with the Lyman-$\alpha$ bounds Boyarsky:08d, and the points with $M_1 \leq 4\;\mathrm{keV}$ are also compatible with X-ray bounds. The region below $1\;\mathrm{keV}$ is ruled out according to the phase-space density arguments Boyarsky:08a. Abbreviation: BBN, big bang nucleosynthesis.