  The six-meter Atacama Cosmology Telescope (ACT) in Chile was built to measure
the cosmic microwave background (CMB) at arcminute angular scales. We are
building a new polarization sensitive receiver for ACT (ACTPol). ACTPol will
characterize the gravitational lensing of the CMB and aims to constrain the sum
of the neutrino masses with ~0.05 eV precision, the running of the spectral
index of inflation-induced fluctuations, and the primordial helium abundance to
better than 1%. Our observing fields will overlap with the SDSS BOSS survey at
optical wavelengths, enabling a variety of cross-correlation science, including
studies of the growth of cosmic structure from Sunyaev-Zel'dovich observations
of clusters of galaxies as well as independent constraints on the sum of the
neutrino masses. We describe the science objectives and the initial receiver
design.
