Light Sterile Neutrinos: Models and Phenomenology

TL;DR

The paper addresses the plausibility of light eV-scale sterile neutrinos by combining model-independent phenomenology with concrete model-building. It analyzes how sterile states modify neutrino mixing, oscillations at short baselines, and the neutrino-less double beta decay observable, providing explicit expressions for the effective mass and mixing in multi-sterile scenarios. It then demonstrates two complementary frameworks—an $A_4$ flavor-symmetry realization that perturb TBM through active-sterile mixing and extended seesaw constructions (including a minimal extension) that naturally yield $m_s\sim$ eV with sizeable mixings. The results highlight that sterile neutrinos can be consistent with current data in certain mass orderings and that upcoming oscillation and beta-decay experiments, together with cosmological observations, will decisively test these possibilities.

Abstract

Motivated by recent hints in particle physics and cosmology, we study the realization of eV-scale sterile neutrinos within both the seesaw mechanism and flavor symmetry theories. We show that light sterile neutrinos can rather easily be accommodated in the popular A_4 flavor symmetry models. The exact tri-bimaximal mixing pattern is perturbed due to active-sterile mixing, which we discuss in detail for one example. In addition, we find an interesting extension of the type I seesaw, which can provide a natural origin for eV-scale sterile neutrinos as well as visible admixtures between sterile and active neutrinos. We also show that the presence of sterile neutrinos would significantly change the observables in neutrino experiments, specifically the oscillation probabilities in short-baseline experiments and the effective mass in neutrino-less double beta decay. The latter can prove particularly helpful to strengthen the case for eV-scale sterile neutrinos.

Figures (6)

• Figure 1: The allowed ranges in the $\langle m_{ee}\rangle-m_{\rm light}$ parameter space, both in the standard three-neutrino picture (unshaded regions) and with one sterile neutrino (shaded regions), for the 1+3 (top) and 3+1 (bottom) cases.
• Figure 2: Same as Fig. \ref{['fig:mee_4nu']}, for the 2+3 (top) and 3+2 (bottom) cases.
• Figure 3: Same as Fig. \ref{['fig:mee_4nu']}, for the 1+3+1a (top) and 1+3+1b (bottom) cases.
• Figure 4: The allowed values in $a-d$ and $a-e$ parameter space for normal (NO) and inverted (IO) ordering, obtained by varying each parameter between $-0.5$ and $0.5$ eV, varying $m_s$ between $-1.5$ and $1.5$ eV, and requiring that the oscillation parameters lie in the correct range Kopp:2011qdSchwetz:2011qt.
• Figure 5: $\sin^2\!\theta_{14}$ against $\sin^2\!\theta_{12}$ and $\sin^2\!\theta_{23}$, for both the normal and inverted ordering. The dashed (black) lines corresponds to the TBM values of $\sin^2\!\theta_{12}$ and $\sin^2\!\theta_{23}$, the solid (red) lines indicate the $2\sigma$ ranges of the parameters and the (red) square is the best-fit point Kopp:2011qdSchwetz:2011qt.