Exploring cosmic origins with CORE: gravitational lensing of the CMB
  Lensing of the CMB is now a well-developed probe of large-scale clustering
over a broad range of redshifts. By exploiting the non-Gaussian imprints of
lensing in the polarization of the CMB, the CORE mission can produce a clean
map of the lensing deflections over nearly the full-sky. The number of high-S/N
modes in this map will exceed current CMB lensing maps by a factor of 40, and
the measurement will be sample-variance limited on all scales where linear
theory is valid. Here, we summarise this mission product and discuss the
science that it will enable. For example, the summed mass of neutrinos will be
determined to an accuracy of 17 meV combining CORE lensing and CMB two-point
information with contemporaneous BAO measurements, three times smaller than the
minimum total mass allowed by neutrino oscillations. In the search for B-mode
polarization from primordial gravitational waves with CORE, lens-induced
B-modes will dominate over instrument noise, limiting constraints on the
gravitational wave power spectrum amplitude. With lensing reconstructed by
CORE, one can "delens" the observed polarization internally, reducing the
lensing B-mode power by 60%. This improves to 70% by combining lensing and CIB
measurements from CORE, reducing the error on the gravitational wave amplitude
by 2.5 compared to no delensing (in the null hypothesis). Lensing measurements
from CORE will allow calibration of the halo masses of the 40000 galaxy
clusters that it will find, with constraints dominated by the clean
polarization-based estimators. CORE can accurately remove Galactic emission
from CMB maps with its 19 frequency channels. We present initial findings that
show that residual Galactic foreground contamination will not be a significant
source of bias for lensing power spectrum measurements with CORE. [abridged]
