First Identification of a CMB Lensing Signal Produced by $1.5$ Million
  Galaxies at $z\sim$4: Constraints on Matter Density Fluctuations at High
  Redshift
  We report the first detection of the dark matter distribution around Lyman
break galaxies (LBGs) at high redshift through the Cosmic Microwave Background
(CMB) lensing measurements with the public {\it Planck} PR3 $\kappa$ map. The
LBG sample consists of 1,473,106 objects with the median redshift of $z \sim 4$
that are identified in a total area of 305 deg$^2$ observed by the Hyper
Suprime-Cam (HSC) Strategic Survey Program (SSP) survey. After careful
investigations of systematic uncertainties, such as contamination from
foreground galaxies and Cosmic Infrared Background (CIB), we obtain the
significant detection of the CMB lensing signal at $5.1\sigma$ that is
dominated by 2-halo term signals of the LBGs. Fitting a simple model consisting
of the Navarro-Frenk-White (NFW) profile and the linear-bias model, we obtain
the typical halo mass of $3.1^{+9.3}_{-2.8} \times 10^{11} h^{-1} M_\odot$.
Combining the CMB lensing and galaxy-galaxy clustering signals on the large
scales, we demonstrate the first cosmological analysis at $z\sim4$ that
constrains $(\Omega_{{\rm m}0}$, $\sigma_8)$. We find that our constraint on
$\sigma_8$ is roughly consistent with the {\it Planck} best-fit cosmology,
while this $\sigma_8$ constraint is lower than the {\it Planck} cosmology over
the $1\sigma$ level. This study opens up a new window for constraining
cosmological parameters at high redshift by the combination of CMB and high-$z$
galaxies as well as studying the interplay between galaxy evolution and
larges-scale structure at such high redshift, by upcoming CMB and optical and
near-infrared imaging surveys.
