Strategies to Detect Dark-Matter Decays with Line-Intensity Mapping
  The nature of dark matter is a longstanding mystery in cosmology, which can
be studied with laboratory or collider experiments, as well as astrophysical
and cosmological observations. In this work, we propose realistic and efficient
strategies to detect radiative products from dark-matter decays with
line-intensity mapping (LIM) experiments. This radiation will behave as a line
interloper for the atomic and molecular spectral lines targeted by LIM surveys.
The most distinctive signatures of the contribution from dark-matter radiative
decays are an extra anisotropy on the LIM power spectrum due to projection
effects, as well as a narrowing and a shift towards higher intensities of the
voxel intensity distribution. We forecast the minimum rate of decays into two
photons that LIM surveys will be sensitive to as function of the dark-matter
mass in the range $\sim 10^{-6}-10$ eV, and discuss how to reinterpret such
results for dark matter that decays into a photon and another particle. We find
that both the power spectrum and the voxel intensity distribution are expected
to be very sensitive to the dark-matter contribution, with the voxel intensity
distribution being more promising for most experiments considered. Interpreting
our results in terms of the axion, we show that LIM surveys will be extremely
competitive to detect its decay products, improving several orders of
magnitudes (depending on the mass) the sensitivity of laboratory and
astrophysical searches, especially in the mass range $\sim 1-10$ eV.
