The Atacama Cosmology Telescope: A CMB lensing mass map over 2100 square
  degrees of sky and its cross-correlation with BOSS-CMASS galaxies
  We construct cosmic microwave background lensing mass maps using data from
the 2014 and 2015 seasons of observations with the Atacama Cosmology Telescope
(ACT). These maps cover 2100 square degrees of sky and overlap with a wide
variety of optical surveys. The maps are signal dominated on large scales and
have fidelity such that their correlation with the cosmic infrared background
is clearly visible by eye. We also create lensing maps with thermal
Sunyaev-Zel'dovich contamination removed using a novel cleaning procedure that
only slightly degrades the lensing signal-to-noise ratio. The cross-spectrum
between the cleaned lensing map and the BOSS CMASS galaxy sample is detected at
$10$-$\sigma$ significance, with an amplitude of $A=1.02 \pm 0.10$ relative to
the Planck best-fit LCDM cosmological model with fiducial linear galaxy bias.
Our measurement lays the foundation for lensing cross-correlation science with
current ACT data and beyond.
