Searching for the Radiative Decay of the Cosmic Neutrino Background with Line-Intensity Mapping

We study the possibility to use line-intensity mapping (LIM) to seek photons from the radiative decay of neutrinos in the cosmic neutrino background. The Standard Model prediction for the rate for these decays is extremely small, but it can be enhanced if new physics increases the neutrino electromagnetic moments. The decay photons will appear as an interloper of astrophysical spectral lines. We propose that the neutrino-decay line can be identified with anisotropies in LIM clustering and also with the voxel intensity distribution. Ongoing and future LIM experiments will have—depending on the neutrino hierarchy, transition, and experiment considered—a sensitivity to an effective electromagnetic transition moment $\sim {10}^{-12}-{10}^{-8}(m_i{c}^2/0.1\mathrm{eV}{)}^{3/2}{\mu }_B$, where $m_i$ is the mass of the decaying neutrino and ${\mu }_B$ is the Bohr magneton. This will be significantly more sensitive than cosmic microwave background spectral distortions, and it will be competitive with stellar cooling studies. As a by-product, we also report an analytic form of the one-point probability distribution function for neutrino-density fluctuations, obtained from the quijote simulations using symbolic regression.

Figure 1

Relation between the sum of neutrino masses and the rest-frame frequency of the photon produced in the decay for NH and IH (top) and the corresponding observed frequency. We also show the frequency bands of the experiments considered in both panels with horizontal shaded bands.

Figure 2

Forecasted 95% confidence level marginalized upper limits of ${\mathrm{\Theta }}_{\nu }$ as a function of the total neutrino mass from measurements for the power spectrum (dashed lines) and VID (solid lines) for all LIM surveys considered, namely, COMAP1 (orange), COMAP2 (red), CCAT-prime (dark yellow), and ATLAST (light blue). The left panels refer to the NH case, while the right ones to the IH case. In both cases we also indicate the considered transition between eigenstates. The dotted purple lines indicate the CMB limits from Ref. [42, 43].