Testing CMB polarization data using position angles
  We consider a novel null test for contamination which can be applied to CMB
polarization data that involves analysis of the statistics of the polarization
position angles. Specifically, we will concentrate on using histograms of the
measured position angles to illustrate the idea. Such a test has been used to
identify systematics in the NVSS point source catalogue with an amplitude well
below the noise level. We explore the statistical properties of polarization
angles in CMB maps. If the polarization angle is not correlated between pixels,
then the errors follow a simple $\sqrt{N_{pix}}$ law. However this is typically
not the case for CMB maps since these have correlations which result in an
increase in the variance since the effective number of independent pixels is
reduced. Then we illustrate how certain classes of systematic errors can result
in very obvious patterns in these histograms, and thus that these errors could
possibly be identified using this method. We discuss how this idea might be
applied in a realistic context, and make a preliminary analysis of the WMAP7
data, finding evidence of a systematic error in the Q and W band data,
consistent with a constant offset in Q and U.
