Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Do solar neutrinos decay?

John F. Beacom, Nicole F. Bell

TL;DR

The paper investigates whether solar neutrinos decay and how to bound such decays in a model-independent way. Framing the problem in the LMA-MSW context, it treats ν2→ν1+X decays as a perturbation and analyzes potential signatures in the SK/SNO spectra, highlighting how mass hierarchy, active vs. sterile daughters, and oscillation degeneracies affect detectability. The main finding is that, despite the well-measured solar flux, the bound on the lifetime-to-mass ratio $τ/m$ for invisible decays is only weakly constrained, of order $≥ 10^{-4}$ s/eV, and is not universal across models. The work emphasizes that improving constraints will require KamLAND to fix oscillation parameters, tighter $^8$B flux measurements, and potentially observations of decay products, while noting that the solar bound is still the strongest direct constraint on non-radiative neutrino decay and does not guarantee absence of decay for astrophysical sources.

Abstract

Despite the fact that the solar neutrino flux is now well-understood in the context of matter-affected neutrino mixing, we find that it is not yet possible to set a strong and model-independent bound on solar neutrino decays. If neutrinos decay into truly invisible particles, the Earth-Sun baseline defines a lifetime limit of $τ/m \agt 10^{-4}$ s/eV. However, there are many possibilities which must be excluded before such a bound can be established. There is an obvious degeneracy between the neutrino lifetime and the mixing parameters. More generally, one must also allow the possibility of active daughter neutrinos and/or antineutrinos, which may partially conceal the characteristic features of decay. Many of the most exotic possibilities that presently complicate the extraction of a decay bound will be removed if the KamLAND reactor antineutrino experiment confirms the large-mixing angle solution to the solar neutrino problem and measures the mixing parameters precisely. Better experimental and theoretical constraints on the $^8$B neutrino flux will also play a key role, as will tighter bounds on absolute neutrino masses. Though the lifetime limit set by the solar flux is weak, it is still the strongest direct limit on non-radiative neutrino decay. Even so, there is no guarantee (by about eight orders of magnitude) that neutrinos from astrophysical sources such as a Galactic supernova or distant Active Galactic Nuclei will not decay.

Do solar neutrinos decay?

TL;DR

The paper investigates whether solar neutrinos decay and how to bound such decays in a model-independent way. Framing the problem in the LMA-MSW context, it treats ν2→ν1+X decays as a perturbation and analyzes potential signatures in the SK/SNO spectra, highlighting how mass hierarchy, active vs. sterile daughters, and oscillation degeneracies affect detectability. The main finding is that, despite the well-measured solar flux, the bound on the lifetime-to-mass ratio for invisible decays is only weakly constrained, of order s/eV, and is not universal across models. The work emphasizes that improving constraints will require KamLAND to fix oscillation parameters, tighter B flux measurements, and potentially observations of decay products, while noting that the solar bound is still the strongest direct constraint on non-radiative neutrino decay and does not guarantee absence of decay for astrophysical sources.

Abstract

Despite the fact that the solar neutrino flux is now well-understood in the context of matter-affected neutrino mixing, we find that it is not yet possible to set a strong and model-independent bound on solar neutrino decays. If neutrinos decay into truly invisible particles, the Earth-Sun baseline defines a lifetime limit of s/eV. However, there are many possibilities which must be excluded before such a bound can be established. There is an obvious degeneracy between the neutrino lifetime and the mixing parameters. More generally, one must also allow the possibility of active daughter neutrinos and/or antineutrinos, which may partially conceal the characteristic features of decay. Many of the most exotic possibilities that presently complicate the extraction of a decay bound will be removed if the KamLAND reactor antineutrino experiment confirms the large-mixing angle solution to the solar neutrino problem and measures the mixing parameters precisely. Better experimental and theoretical constraints on the B neutrino flux will also play a key role, as will tighter bounds on absolute neutrino masses. Though the lifetime limit set by the solar flux is weak, it is still the strongest direct limit on non-radiative neutrino decay. Even so, there is no guarantee (by about eight orders of magnitude) that neutrinos from astrophysical sources such as a Galactic supernova or distant Active Galactic Nuclei will not decay.

Paper Structure

This paper contains 10 sections, 19 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Neutrino Decay Models
  3. Solar $\nu_2$ Decay
  4. Simple $\nu_2$ Disappearance
  5. Compensating Effects from Oscillations
  6. Appearance of Active Daughter $\nu_1$
  7. Dark-Side Inversion Undone by Decay
  8. Other Possibilities
  9. Other Neutrino Decays
  10. Conclusions

Figures (5)

  • Figure 1: The values of the neutrino mass eigenvalues as a function of the unknown smallest mass $m_1$. When $m_1$ is specified, the measured solar and atmospheric $\delta m^2$ values fix $m_2$ and $m_3$. For an inverted hierarchy ($\delta m^2_{atm} < 0$), the two upper eigenvalues are very nearly degenerate at the position of the curve labeled $m_3$. The present bound on neutrino mass from tritium experiments is about 2 eV tritium; by the construction above, it applies to all three mass eigenvalues.
  • Figure 2: In the upper panel, the electron neutrino survival probability is shown versus neutrino energy. In the lower panel, the ratio of measured to expected spectra in SK is shown versus the recoil electron total energy, as detected in neutrino-electron scattering above 5 MeV. The solid lines correspond to the LMA solution ($\delta m^2 = 4 \times 10^{-5}$ eV$^2$, $\sin^2{2\theta} = 0.9$) with stable $\nu_2$. The dashed lines, in order of decreasing height, correspond to $\nu_2$ lifetimes of $\tau/m = 10^{-3}, 10^{-4},{\rm and\ } 10^{-5}$ s/eV (by Eq. (\ref{['edecay']}), these correspond to decays at typical energies of 0.5, 5, and 50 MeV, respectively). The 1258-day data from SK are shown in the lower panel, as is the size of the flux normalization uncertainty. The decay products of $\nu_2$ are considered to be sterile.
  • Figure 3: An example of the parameter degeneracy between the lifetime and the mixing parameters is illustrated. The description is as for Fig. \ref{['basic']}, except that now $\delta m^2 = 12 \times 10^{-5}$ eV$^2$, which flattens $R_{SK}$ for $\tau/m = 10^{-4}$ s/eV, making it consistent with the SK spectral shape. It is about $1\sigma$ discrepant in terms of normalization.
  • Figure 4: The effect of $\nu_2$ decay to active $\nu_1$ daughters is shown. We have assumed $m_1$ and $m_2$ are nearly degenerate, so that the daughter energy is approximately the full energy of the parent. The description is as for Fig. \ref{['basic']}, except that now the dotted lines, in order of increasing height, correspond to $\nu_2$ lifetimes of $\tau/m = 10^{-3}, 10^{-4},{\rm and\ } 10^{-5}$ s/eV. Again, the case $\tau/m = 10^{-4}$ s/eV is discrepant in terms of shape and normalization.
  • Figure 5: The effect of $\nu_2$ decay to active $\nu_1$ daughters is shown again, this time with a "dark-side" angle of $54^\circ$ instead of $36^\circ$ (but still with $\delta m^2 = 4 \times 10^{-5}$ eV$^2$, $\sin^2{2\theta} = 0.9$). The description is as for Fig. \ref{['basic']}, except that now the dotted lines, in order of decreasing height, correspond to $\nu_2$ lifetimes of $\tau/m = 10^{-3}, 10^{-4},{\rm and\ } 10^{-5}$ s/eV. In this unusual case, a lifetime as short as$10^{-5}$ s/eV is required to undo the $\nu_2/\nu_1$ reversal in $\nu_e$ caused by choosing a "dark-side" angle.