Robust CMB B-mode analysis with Needlet-ILC and simulation-based inference

TL;DR

This paper introduces a new analysis framework combining simulation-based inference, NILC, and cross-correlation statistics to improve the robustness and accuracy of CMB B-mode polarization measurements, especially for the tensor-to-scalar ratio.

Contribution

The novel framework integrates NILC and simulation-based inference with neural posterior estimation to better handle foreground contamination in CMB polarization data.

Findings

Improved statistical constraints on the tensor-to-scalar ratio r.

Demonstrated unbiased r results across complex foreground simulations.

Enhanced robustness to foreground complexity compared to existing methods.

Abstract

We explore a novel analysis framework for parameter inference with large-scale CMB polarization data. Our method uses simulation-based inference combined with the needlet internal linear combination (NILC) algorithm and cross-correlation-based statistics to compress the data into a vector that is robust to model misspecification and small enough to be amenable to neural posterior estimation with normalizing flows. By leveraging this compressed data representation, our method enables the robust use of the anisotropic and non-Gaussian information in the foreground fields to more accurately separate the CMB polarization signal from these contaminants. Using an idealized ground-based experimental setup inspired by the Simons Observatory Small Aperture Telescopes, we demonstrate improved statistical constraining power for the tensor-to-scalar ratio $r$ compared to the (constrained) NILC algorithm and improved robustness to complex foregrounds compared to other techniques in the literature. Trained on a relatively simple semi-analytical foreground model, the method yields unbiased $r$ results across a range of PySM Galactic foreground simulations, including the high-complexity d12 model, for which we obtain $r=(1.09 \pm 0.27)\cdot 10^{-2}$ for input $r=0.01$ and sky fraction $f_{\mathrm{sky}} = 0.21$. We thus demonstrate the feasibility and advantages of a complete, maps-to-parameters, simulation-based analysis of large-scale CMB polarization for current ground-based observatories.