Cosmic Microwave Background Delensing Revisited: Residual Biases and a Simple Fix
Abstract
The delensing procedure is an effective tool for removing lensing-induced B-mode polarization in the Cosmic Microwave Background to allow for deep searches of primordial B-modes. However, the delensing algorithm existing in the literature breaks down if the target B-mode signals overlap significantly with the lensing B-mode ( 300<l<2000) in multipole-l space. In this paper, we identify the cause of the breakdown to be correlations between the input B-map and the deflection field estimator (EB). The amplitude of this bias is quantified by numerical simulations and compared to the analytically derived functional form. We also propose a revised delensing procedure that circumvents the bias. While the newly identified bias does not affect the search of degree scale B-mode generated by tensor perturbations, the modified delensing algorithm makes it possible to perform deep searches of high-l B-modes such as those generated by patchy reionization, cosmic strings, or rotation of polarization angles. Finally, we estimate how well future polarization experiments can do in detecting tensor- and cosmic string- generated B-mode after delensing and comment on different survey strategies.