Decomposing CMB lensing power with simulation

TL;DR

This paper analyzes the limitations of current CMB lensing reconstruction methods, extends the analysis to polarization, and assesses the impact of higher order terms on future experimental sensitivities.

Contribution

It generalizes the analysis of higher order effects to all quadratic estimators and evaluates the sensitivity of mitigation techniques to spectral uncertainties.

Findings

Higher order terms affect future experimental error estimates.

Modified estimators rely on precise knowledge of lensed spectra.

Higher order effects become significant at advanced experimental sensitivities.

Abstract

The reconstruction of the CMB lensing potential is based on a Taylor expansion of lensing effects which is known to have poor convergence properties. For lensing of temperature fluctuations, an understanding of the higher order terms in this expansion which is accurate enough for current experimental sensitivity levels has been developed in Hanson et. al. (2010), as well as a slightly modified Okamoto and Hu quadratic estimator which incorporates lensed rather than unlensed spectra into the estimator weights to mitigate the effect of higher order terms. We extend these results in several ways: (1) We generalize this analysis to the full set of quadratic temperature/polarization lensing estimators, (2) We study the effect of higher order terms for more futuristic experimental noise levels, (3) We show that the ability of the modified quadratic estimator to mitigate the effect of higher order terms relies on a delicate cancellation which occurs only when the true lensed spectra are known. We investigate the sensitivity of this cancellation to uncertainties in or knowledge of these spectra. We find that higher order terms in the Taylor expansion can impact projected error bars at experimental sensitivities similar to those found in future ACTpol/SPTpol experiments.