  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 $\sim 10^{-12}\, -\,10^{-8}\,
(m_ic^2/{0.1 \rm eV})^{3/2}\mu_{\rm B}$, where $m_i$ is the mass of the
decaying neutrino and $\mu_{\rm B}$ 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
byproduct, 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.
