Phenomenology of light sterile neutrinos: a brief review

TL;DR

This review addresses anomalous results at very short baselines that challenge the standard 3-neutrino paradigm and discusses their interpretation in a minimal $3+1$ sterile-neutrino framework with mass-squared splitting $\Delta m^2_{14}$ of order $1\ \mathrm{eV}^2$ and small mixings $|U_{e4}|^2$, $|U_{\mu4}|^2$. It surveys accelerator (LSND, MiniBooNE, ICARUS), reactor/gallium, and cosmological (dark radiation) anomalies, and analyzes the tension between appearance and disappearance data as well as cosmological mass bounds. It shows that while the $3+1$ scenario is attractive for its simplicity, substantial inconsistencies remain, and independent tests (e.g., solar+KamLAND with dual-baseline reactors) place stringent limits on $|U_{e4}|^2$ and reduce the statistical significance of hints for $\theta_{14}>0$. The authors advocate new, highly sensitive experiments to either discover sterile-neutrino oscillations or definitively rule out light sterile states, with cosmological data (e.g., Planck) providing complementary constraints on the viability of these models.

Abstract

An increasing number of anomalous experimental results are emerging, which cannot be described within the standard 3-neutrino framework. We present a concise discussion of the most popular phenomenological interpretation of such findings, based on a hypothetical flavor conversion phenomenon of the ordinary "active" neutrinos into new light "sterile" species having mass m ~ O(1) eV.

Figures (5)

• Figure 1: Regions allowed by the main published experiments sensitive to the accelerator anomaly superimposed to the limits established by the ICARUS experiment. Figure taken fromAntonello:2012pq.
• Figure 2: Regions allowed by the combination of the reactor and gallium anomalies (figure taken fromAbazajian:2012ys).
• Figure 3: Regions allowed by the CMB data. The ordinary neutrinos are assumed to be massless while the $N_s$ sterile species have a common mass $m_s$ (figure taken fromHamann:2010bk).
• Figure 4: The solid lines delimit the $3\sigma$ regions allowed respectively by the appearance and disappearance data, while the colored regions are those preferred by the combination of the two datasets (figure taken fromGiunti:2011cp).
• Figure 5: Left panel: regions allowed by the solar sector data (diagonal bands) and by the dual-baseline reactor (Daya Bay and RENO) experiments (vertical bands). Middle panel: regions allowed by their combination. Right panel: combination of the constraints in the middle panel with those coming from the reactor anomaly. The contours refer to $\Delta \chi^2 =1$ (dotted line) and $\Delta \chi^2 = 4$ (solid line).