Light sterile neutrinos

TL;DR

The review investigates whether light sterile neutrinos at the eV scale can account for short-baseline anomalies (reactor, Gallium, LSND) by extending the standard three-neutrino framework to 3+1/3+2 schemes. It analyzes short-baseline oscillation probabilities, fits to data, and implications for beta decay and neutrinoless double-beta decay, alongside cosmological constraints from Planck and large-scale structure. A persistent tension between appearance and disappearance data emerges, and cosmology disfavors fully thermalized eV-scale sterile states, though partial thermalization or non-standard production can soften the conflict. The paper highlights a broad experimental program (source, reactor, accelerator) and near-term cosmological and lab-based tests (e.g., MicroBooNE, KATRIN) as crucial to determining the viability of light sterile neutrinos and their role in new physics beyond the Standard Model.

Abstract

The theory and phenomenology of light sterile neutrinos at the eV mass scale is reviewed. The reactor, Gallium and LSND anomalies are briefly described and interpreted as indications of the existence of short-baseline oscillations which require the existence of light sterile neutrinos. The global fits of short-baseline oscillation data in 3+1 and 3+2 schemes are discussed, together with the implications for beta-decay and neutrinoless double-beta decay. The cosmological effects of light sterile neutrinos are briefly reviewed and the implications of existing cosmological data are discussed. The review concludes with a summary of future perspectives.

Figures (7)

• Figure 1: Schematic illustration of the 3+1, 3+2 and 3+1+1 neutrino mixing schemes taking into account for each scheme the two possible mass ordering, normal (NO) and inverted (IO), of the three lightest standard neutrinos. In the two 3+2 schemes $\Delta m^2_{54} \approx \Delta m^2_{\text{SBL}}$, whereas in the two 3+1+1 schemes $\Delta m^2_{54} \gg \Delta m^2_{\text{SBL}}$.
• Figure 2: Ratios $R$ of the measured ($N_{\text{exp}}$) and calculated ($N_{\text{cal}}$) number of electron antineutrino events in reactor experiments at different distances $L$. The horizontal shadowed red band shows the average ratio $\overline{R}$ and its uncertainty. For each experiment the error bar shows the experimental uncertainty. The values of the ratios of the long-baseline experiments Daya Bay, Double Chooz, Chooz and Palo Verde have been obtained by subtracting the effect of $\vartheta_{13}$-driven oscillations.
• Figure 3: \ref{['fig:GaGe']}: ${}^{71}\text{Ga}\to{}^{71}\text{Ge}$ transitions induced by ${}^{51}\text{Cr}$ and ${}^{37}\text{Ar}$ electron neutrinos. \ref{['fig:gal']}: Ratios $R$ of the measured ($N_{\text{exp}}$) and calculated ($N_{\text{cal}}$) number of electron neutrino events in the GALLEX and SAGE radioactive source experiments. The horizontal shadowed red band shows the average ratio $\overline{R}$ and its uncertainty. For each experiment the error bar shows the experimental uncertainty.
• Figure 4: Allowed regions in the $\sin^{2}2\vartheta_{e\mu}$--$\Delta{m}^{2}_{41}$, $\sin^{2}2\vartheta_{ee}$--$\Delta{m}^{2}_{41}$ and $\sin^{2}2\vartheta_{\mu\mu}$--$\Delta{m}^{2}_{41}$ planes obtained in the pragmatic 3+1-PrGLO global fit of short-baseline neutrino oscillation data compared with the $3\sigma$ allowed regions obtained from $\hbox{$\overset{(-)}{\space}$}{{\nu}_{\mu}}\xspace\to\hbox{$\overset{(-)}{\space}$}{{\nu}_{e}}\xspace$ short-baseline appearance data (APP) and the $3\sigma$ constraints obtained from $\hbox{$\overset{(-)}{\space}$}{{\nu}_{e}}\xspace$ short-baseline disappearance data ($\nu_{e}$ DIS), $\hbox{$\overset{(-)}{\space}$}{{\nu}_{\mu}}\xspace$ short-baseline disappearance data ($\nu_{\mu}$ DIS) and the combined short-baseline disappearance data (DIS). The best-fit points of the PrGLO and APP fits are indicated by crosses.
• Figure 5: \ref{['fig:mainz']}: Upper 90% C.L. limit on $\sin^2(\vartheta)=|U_{e4}|^2$ as a function of $m(\nu_{4})^2=m_{4}^2$ obtained in the Mainz experiment Kraus:2012he. \ref{['fig:troitsk']}: Upper 95% C.L. limits on $U_{e4}^2=|U_{e4}|^2$ as functions of $m_{4}^2$ obtained with different statistical methods in the Troitsk experiment Belesev:2013cba.