Systematic Effects in CMB Polarization Measurements
  The cosmic microwave background polarization is rich of cosmological information complementary to those from temperature anisotropies. Linear polarization can be decomposed uniquely in two components of opposite parities, called E and B. While E mode allows measurement of cosmological parameters in a way independent from temperature, B mode allows to detect the primordial gravitational waves produced during inflation, and thus to determine its energy scale. However, measuring CMB polarization is complicated by foregrounds, whose polarization is poorly known, and by systematic effects, which mainly affects B mode measurement because of its low level. As an example, we show here the effect of beams uncertainty on polarization measurement in the case of the Planck HFI instrument, and how we can correct for it. 
C. Rosset (PCC), the PLANCK-HFI Collaboration