In-flight polarization angle calibration for LiteBIRD: blind challenge
  and cosmological implications
  We present a demonstration of the in-flight polarization angle calibration
for the JAXA/ISAS second strategic large class mission, LiteBIRD, and estimate
its impact on the measurement of the tensor-to-scalar ratio parameter, r, using
simulated data. We generate a set of simulated sky maps with CMB and polarized
foreground emission, and inject instrumental noise and polarization angle
offsets to the 22 (partially overlapping) LiteBIRD frequency channels. Our
in-flight angle calibration relies on nulling the EB cross correlation of the
polarized signal in each channel. This calibration step has been carried out by
two independent groups with a blind analysis, allowing an accuracy of the order
of a few arc-minutes to be reached on the estimate of the angle offsets. Both
the corrected and uncorrected multi-frequency maps are propagated through the
foreground cleaning step, with the goal of computing clean CMB maps. We employ
two component separation algorithms, the Bayesian-Separation of Components and
Residuals Estimate Tool (B-SeCRET), and the Needlet Internal Linear Combination
(NILC). We find that the recovered CMB maps obtained with algorithms that do
not make any assumptions about the foreground properties, such as NILC, are
only mildly affected by the angle miscalibration. However, polarization angle
offsets strongly bias results obtained with the parametric fitting method. Once
the miscalibration angles are corrected by EB nulling prior to the component
separation, both component separation algorithms result in an unbiased
estimation of the r parameter. While this work is motivated by the conceptual
design study for LiteBIRD, its framework can be broadly applied to any CMB
polarization experiment. In particular, the combination of simulation plus
blind analysis provides a robust forecast by taking into account not only
detector sensitivity but also systematic effects.
