Towards ending the partial sky E-B ambiguity in CMB observations

A crucial problem for partial sky analysis of CMB polarization is the E-B leakage problem. Such leakage arises from the presence of 'ambiguous' modes that satisfy properties of both E and B modes. Solving this problem is critical for primordial polarization B mode detection in partial sky CMB polarization experiments. In this work we introduce a new method for reducing the leakage. We demonstrate that if we complement the E-mode information outside the observation patch with ancillary data from full-sky CMB observations, we can reduce and even effectively remove the E-to-B leakage. For this objective, we produce E-mode Stokes QU maps from Wiener filtered full-sky intensity and polarization CMB observations. We use these maps to fill the sky region that is not observed by the ground-based experiment of interest, and thus complement the partial sky Stokes QU maps. Since the E-mode information is now available on the full sky we see a significant reduction in the E-to-B leakage. We evaluate on simulated data sets the performance of our method for a 'shallow' f(sky )= 8%, and a 'deep' f(sky) = 2% northern hemisphere sky patch, with AliCPT-like properties, and a LSPE-like f(sky )= 30% sky patch, by combining those observations with Planck-like full sky polarization maps. We find that our method outperforms the standard and the pure-B method pseudo-C-l estimators for all of our simulations. Our new method gives unbiased estimates of the B-mode power spectrum through-out the entire multipole range with near-optimal pseudo-C-l errors for l > 20. We also study the application of our method to the CMB-54 experiment combined with LiteBIRD-like full sky data, and show that using signal-dominated full sky E-mode data we can eliminate the E-to-B leakage problem.

Automated summary

Introducing a new method to reduce E-B leakage in partial sky CMB polarization experiments by complementing with full-sky data has shown significant success in eliminating the leakage problem, improving B-mode detection efficiency.