The Atacama Cosmology Telescope: CMB Polarization at $200<\ell<9000$
  We report on measurements of the cosmic microwave background (CMB) and
celestial polarization at 146 GHz made with the Atacama Cosmology Telescope
Polarimeter (ACTPol) in its first three months of observing. Four regions of
sky covering a total of 270 square degrees were mapped with an angular
resolution of $1.3'$. The map noise levels in the four regions are between 11
and 17 $\mu$K-arcmin. We present TT, TE, EE, TB, EB, and BB power spectra from
three of these regions. The observed E-mode polarization power spectrum,
displaying six acoustic peaks in the range $200<\ell<3000$, is an excellent fit
to the prediction of the best-fit cosmological models from WMAP9+ACT and Planck
data. The polarization power spectrum, which mainly reflects primordial plasma
velocity perturbations, provides an independent determination of cosmological
parameters consistent with those based on the temperature power spectrum, which
results mostly from primordial density perturbations. We find that without
masking any point sources in the EE data at $\ell<9000$, the Poisson tail of
the EE power spectrum due to polarized point sources has an amplitude less than
$2.4$ $\mu$K$^2$ at $\ell = 3000$ at 95\% confidence. Finally, we report that
the Crab Nebula, an important polarization calibration source at microwave
frequencies, has 8.7\% polarization with an angle of $150.7^\circ \pm
0.6^\circ$ when smoothed with a $5'$ Gaussian beam.
