  A millimeter-wave survey over half the sky, that spans frequencies in the
range of 30 to 350 GHz, and that is both an order of magnitude deeper and of
higher-resolution than currently funded surveys would yield an enormous gain in
understanding of both fundamental physics and astrophysics. By providing such a
deep, high-resolution millimeter-wave survey (about 0.5 uK-arcmin noise and 15
arcsecond resolution at 150 GHz), CMB-HD will enable major advances. It will
allow 1) the use of gravitational lensing of the primordial microwave
background to map the distribution of matter on small scales (k~10/hMpc), which
probes dark matter particle properties. It will also allow 2) measurements of
the thermal and kinetic Sunyaev-Zel'dovich effects on small scales to map the
gas density and gas pressure profiles of halos over a wide field, which probes
galaxy evolution and cluster astrophysics. In addition, CMB-HD would allow us
to cross critical thresholds in fundamental physics: 3) ruling out or detecting
any new, light (< 0.1eV), thermal particles, which could potentially be the
dark matter, and 4) testing a wide class of multi-field models that could
explain an epoch of inflation in the early Universe. Such a survey would also
5) monitor the transient sky by mapping the full observing region every few
days, which opens a new window on gamma-ray bursts, novae, fast radio bursts,
and variable active galactic nuclei. Moreover, CMB-HD would 6) provide a census
of planets, dwarf planets, and asteroids in the outer Solar System, and 7)
enable the detection of exo-Oort clouds around other solar systems, shedding
light on planet formation. CMB-HD will deliver this survey in 5 years of
observing half the sky, using two new 30-meter-class off-axis cross-Dragone
telescopes to be located at Cerro Toco in the Atacama Desert. The telescopes
will field about 2.4 million detectors (600,000 pixels) in total.
