Blind mitigation of foreground-induced biases on primordial $B$ modes for ground-based CMB experiments

TL;DR

This paper presents two novel extensions to the NILC framework that effectively reduce foreground biases in CMB B-mode polarization measurements, improving the reliability of primordial gravitational wave detection.

Contribution

The work introduces deprojection of foreground moments and likelihood marginalisation within NILC, enhancing foreground mitigation for ground-based CMB experiments.

Findings

Both methods effectively control residual foreground contamination.

Unbiased estimates of the tensor-to-scalar ratio r are achieved.

The approaches enable consistent reconstruction of lensing B-mode amplitude.

Abstract

Observations of the Cosmic Microwave Background (CMB) B-mode polarisation provide a unique probe of inflationary physics. Extracting a reliable constraint on the tensor-to-scalar ratio $r$ nonetheless demands stringent suppression of diffuse Galactic foregrounds, whose residuals can bias the inferred signal. This work introduces and evaluates two extensions of the Needlet Internal Linear Combination (NILC) framework aimed at reducing foreground-induced biases on $r$. The first extension implements the deprojection of selected foreground moments directly within the component-separation step. The second performs a likelihood-level marginalisation over residual foreground power using a data-driven template. Using Simons Observatory Small Aperture Telescope (SO-SAT) - like simulations, we show that both methods effectively control residual contamination, yielding unbiased estimates of $r$ and a consistent reconstruction of the lensing B-mode amplitude. These results indicate that enhanced foreground-mitigation strategies will be useful for next-generation CMB polarisation analyses seeking a robust detection of primordial B-modes.