Extragalactic foregrounds are known to generate significant biases in
temperature-based CMB lensing reconstruction. Several techniques, which include
``source hardening'' and ``shear-only estimators'' have been proposed to
mitigate contamination and have been shown to be very effective at reducing
foreground-induced biases. Here we extend both techniques to polarization,
which will be an essential component of CMB lensing reconstruction for future
experiments, and investigate the ``large-lens'' limit analytically to gain
insight on the origin and scaling of foreground biases, as well as the
sensitivity to their profiles.Using simulations of polarized point sources, we
estimate the expected bias to both Simons Observatory and CMB-S4 like
(polarization-based) lensing reconstruction, finding that biases to the former
are minuscule while those to the latter are potentially non-negligible at small
scales ($L\sim 1000-2000$ ). In particular, we show that for a CMB-S4 like
experiment, an optimal linear combination of point-source hardened estimators
can reduce the (point-source induced) bias to the CMB lensing power spectrum by
up to two orders of magnitude, at a $\sim 4\mathrm{\%}$ noise cost relative to the global
minimum variance estimator.

PREPRINT

# Foreground-immune CMB lensing reconstruction with polarization

Noah Sailer, Simone Ferraro, Emmanuel Schaan

Submitted on 7 November 2022

## Abstract

## Preprint

Comment: 18 pages, 7 figures

Subject: Astrophysics - Cosmology and Nongalactic Astrophysics