A Detection of CMB-Cluster Lensing using Polarization Data from SPTpol
  We report the first detection of gravitational lensing due to galaxy clusters
using only the polarization of the cosmic microwave background (CMB). The
lensing signal is obtained using a new estimator that extracts the lensing
dipole signature from stacked images formed by rotating the cluster-centered
Stokes $Q/U$ map cutouts along the direction of the locally measured background
CMB polarization gradient. Using data from the SPTpol 500 deg$^{2}$ survey at
the locations of roughly 18,000 clusters with richness $\lambda \ge 10$ from
the Dark Energy Survey (DES) Year-3 full galaxy cluster catalog, we detect
lensing at $4.8\sigma$. The mean stacked mass of the selected sample is found
to be $(1.43 \pm 0.4)\ \times 10^{14}\ {\rm M_{\odot}}$ which is in good
agreement with optical weak lensing based estimates using DES data and
CMB-lensing based estimates using SPTpol temperature data. This measurement is
a key first step for cluster cosmology with future low-noise CMB surveys, like
CMB-S4, for which CMB polarization will be the primary channel for cluster
lensing measurements.
