Searching for eV-mass Axion-like Particles with Cross Correlations
  between Line Intensity and Weak Lensing Maps
  We study cross correlations between line intensity and weak lensing maps to
search for axion-like particles (ALPs). Radiative decay of eV-mass ALPs can
contribute to cosmic background emissions at optical and infrared wavelengths.
Line intensity mapping is a unique means of measuring the background emission
at a given photon frequency. If ALPs constitute the abundance of cosmic dark
matter, line intensity maps can correlate with large-scale structures probed by
weak gravitational lensing effects in galaxy imaging surveys. We develop a
theoretical framework to predict the cross correlation. We then explore
potentiality of probing ALPs with the cross correlation in upcoming
galaxy-imaging and spectral surveys. Assuming SPHEREx and the Vera Rubin
Observatory's Legacy Survey of Space and Time (LSST), we find that the cross
correlation by the ALP decay can be greater than the astrophysical-line
counterparts at wavelength of $\sim3000\, {\rm nm}$ for ALPs with a particle
mass of $m_a\sim1\, \mathrm{eV}$ and a particle-to-two-photons coupling of
$g_{a\gamma\gamma}\sim1\times10^{-11}\, \mathrm{GeV}^{-1}$. We also predict
that a null detection of the cross correlation can place a $2\sigma$-level
upper bound of $g_{a\gamma\gamma} \lower.5ex\hbox{$\; \buildrel < \over \sim
\;$} 10^{-11}\, \mathrm{GeV}^{-1}$ for eV-mass ALPs, improving the current
constraint by a factor of $\sim10$. We then make a forecast of expected
constraints on ALP parameters in SPHEREx and LSST by Fisher analysis, providing
a guideline of searching for the ALP decay with the large-scale structure data.
