Statistics of non-polarized points in the CMB polarization maps
  The non-polarized points (NPP) of the $Q$ and $U$ Stokes parameters of the
CMB can be classified according to the geometry of the polarization field. We
describe a procedure to identify these points in the pixelized sky and present
the shape of the polarization angles in the vicinity of NPPs. We design a test
of Gaussianity using the Kullback-Leibler divergence. We show that the total
number density of non-polarized points of the E- and B-families is closely
related to the presence of lensing and the tensor-to-scalar ratio $r$. We
further show that in the absence of lensing, the total number of NPPs of all
types does not depend on $r$, while the lensing effect removes this degeneracy.
This analysis is applied to the CMB maps from the 2018 Planck release. We show
that there is general consistency of SMICA and NILC maps compared to a
reference set of Gaussian simulations. The strongest discrepancies are found in
the Commander (with corresponding $p$-value $0.07$) and NILC ($p = 0.15$) maps.
