Neutrino Phenomenology of Very Low-Energy Seesaws

TL;DR

The paper analyzes the phenomenology of very low-energy seesaws with three right-handed neutrinos, focusing on Majorana masses $M$ at or below the keV scale and their implications for neutrino oscillations, pulsar kicks, and supernova nucleosynthesis. It shows that a 3 active plus 3 sterile framework can accommodate LSND-like signals via a $3+2$ sterile sector with a keV-scale $m_6$, while also addressing astrophysical puzzles, provided cosmological evolution avoids thermalization of the light sterile states. The work emphasizes that the same sterile states can be probed by future tritium beta-decay and neutrinoless double-beta decay experiments, with distinctive signatures such as spectral kinks and altered effective Majorana masses, and argues that experimental tests in the near future will strongly constrain or reveal this low-energy seesaw parameter space. Overall, while appealing for its testability and unification of multiple anomalies, the scenario faces tension with standard cosmology and SN1987A bounds, requiring either nonstandard early-un universe histories or careful tuning of mixings and masses. The paper thus highlights a concrete, experimentally accessible alternative to high-scale seesaw models and maps clear pathways for falsification or confirmation in upcoming experiments.

Abstract

The Standard Model augmented by the presence of gauge-singlet right-handed neutrinos proves to be an ideal scenario for accommodating nonzero neutrino masses. Among the new parameters of this ``New Standard Model'' are right-handed neutrino Majorana masses M. Theoretical prejudice points to M much larger than the electroweak symmetry breaking scale, but it has recently been emphasized that all M values are technically natural and should be explored. Indeed, M around 1-10 eV can accommodate an elegant oscillation solution to the LSND anomaly, while other M values lead to several observable consequences. We consider the phenomenology of low energy seesaw scenarios with M less than and equal to approximately 1 keV. By exploring such a framework with three right-handed neutrinos, we can consistently fit all oscillation data -- including those from LSND -- while partially addressing several astrophysical puzzles, including anomalous pulsar kicks, heavy element nucleosynthesis in supernovae, and the existence of warm dark matter. Furthermore, low-energy seesaws -- regardless of their relation to the LSND anomaly -- can also be tested by future tritium beta-decay experiments, neutrinoless double-beta decay searches, and other observables. We estimate the sensitivity of such probes to M.

Figures (5)

• Figure 1: Neutrino mass eigenstate spectrum, along with the flavor composition of each state. This case accommodates all neutrino oscillation data, constraints from r-process nucleosynthesis in supernovae, and may help explain anomalous pulsar kicks (see text for details). While we choose to depict a normal hierarchy for the active neutrino states, an inverted active neutrino mass hierarchy would have yielded exactly the same physics (as far as the observables considered are concerned).
• Figure 2: Adapted from kicks_gen. Cosmological and astrophysical constraints on the $|U_{\alpha 6}|^2\times m_6$-plane. In the large dark grey region, the density of a thermal $\nu_6$ population is $\Omega_s>0.3$, while the light grey 'X-ray' region is disfavored by X-ray observations. The regions labeled 1,2,3 are preferred if one is to explain anomalous pulsar kicks with active--sterile oscillations inside supernovae. Regions 1 and 3 qualitatively extend inside the $\Omega_s>0.3$ part of the plane as indicated by the horizontal dotted and dash-dotted lines, respectively. The regions 'Warm Dark Matter" and "Too Warm Dark Matter" are meant to represent the region of parameter space where thermal $\nu_6$ qualifies as a good (or bad) warm dark matter candidate. The region above the solid diagonal line is disfavored by the observation of electron (anti)neutrinos from SN1987A. The diagonal dashed lines correspond to $U_{e6}^2=m_l/m_6$, for different values of $m_l$. Also shown is our "best fit" sterile solution for different pulsar kick scenarios, assuming the $3+2$ LSND fit for the lighter states. The regions one and three best fit values are represented by circles and a star, respectively. See text for details.
• Figure 3: $1-S/S_0$ as a function of the $\beta$-ray energy, where $S$ is the $\beta$-ray energy spectrum obtained assuming three mostly active, degenerate neutrinos with mass $m=0.1$ eV and one mostly sterile neutrino $\nu_i$ with $m_i=0.1,0.5,1,5,$ and $10$ eV. The mixing angle is given by $U_{ei}^2=m/m_i$. $S_0$ is the spectrum associated with massless neutrinos. See text for details.
• Figure 4: Contour plot of constant $R$, as defined by Eq. (\ref{['equ:R']}), assuming an energy window $\Delta E = 25~\rm{eV}$. The solid (dashed) line corresponds to $\sqrt{m_l/m_i}$, a naive upper bound for $|U_{ei}|$, for $m_l=0.3$ eV (0.01 eV). The circles correspond to $U_{ei}$ for the three mostly sterile states obtained by our "fit" to other neutrino data, Eq. (\ref{['U_32']}). See text for details.
• Figure 5: Effective $m_{ee}$ for neutrinoless double-beta decay as a function of $m_6$, the heaviest right-handed neutrino mass, assuming the existence of only light, active, neutrinos (magenta curve), with a degenerate mass spectra, and for our "best fit" $3+2$ LSND sterile neutrino solution (blue curve). See text for details. Also indicated is the parameter region preferred by astrophysical hints of sterile neutrinos. We assume $Q = 50~\rm{MeV}$. In the case of a low-energy seesaw, $m_{ee}$ vanishes as long as $m_6\ll Q$.