Spin characterization of systematics in CMB surveys - a comprehensive formalism

The CMB B-mode polarization signal - both the primordial gravitational wave signature and the signal sourced by lensing - is subject to many contaminants from systematic effects. Of particular concern are systematics that result in mixing of signals of different 'spin', particularly leakage from the much larger spin-0 intensity signal to the spin-2 polarization signal. We present a general formalism, which can be applied to arbitrary focal plane setups, that characterizes signals in terms of their spin. We provide general expressions to describe how spin-coupled signals observed by the detectors manifest at map-level, in the harmonic domain, and in the power spectra, focusing on the polarization spectra - the signals of interest for upcoming CMB surveys. We demonstrate the presence of a previously unidentified cross-term between the systematic and the intrinsic sky signal in the power spectrum, which in some cases can be the dominant source of contamination. The formalism is not restricted to intensity to polarization leakage but provides a complete elucidation of all leakage including polarization mixing, and applies to both full and partial (masked) sky surveys, thus covering space-based, balloon-borne, and ground-based experiments. Using a pair-differenced setup, we demonstrate the formalism by using it to completely characterize the effects of differential gain and pointing systematics, incorporating both intensity leakage and polarization mixing. We validate our results with full time ordered data simulations. Finally, we show in an Appendix that an extension of simple binning map-making to include additional spin information is capable of removing spin-coupled systematics during the map-making process.

Automated summary

The study of the CMB B-mode polarization signal requires careful consideration of systematic effects that may contaminate the signals, particularly the mixing of signals of different spin being a significant source of contamination. A general formalism is presented to characterize signals in terms of their spin, providing expressions to describe how spin-coupled signals manifest at various levels of analysis. The presence of a previously unidentified cross-term between systematic and intrinsic sky signals in the power spectrum is shown, with potential implications for contamination in some cases.