To me, the fact that ExB and TxB are (mostly) consistent with zero is a good argument in favour of BICEP2 seeing primordial fluctuations rather than foregrounds. One would expect that a generic foreground would push roughly the same amount of power into all cross-correlations. However, some people insist that ExB=0 by construction because of this paragraph:
Ok, they have one degree of freedom and they can minimize some power. Say a "loop" produces a uniform polarization contribution, that wold be a k=0 mode and you could get rid of that that way. But it is one mode and you definitely cannot kill all correlations with just one d.o.f.. So, is ExB=TxB=0 a good argument in favour of "It cannot all be foregrounds."?Paper, Secition 8: wrote: Once differential ellipticity has been corrected we notice
that an excess of TxB and ExB power remains at > 200 versus
the ΛCDM expectation. The spectral form of this power is
consistent with an overall rotation of the polarization angle of
the experiment. While the detector-to-detector relative angles
have been measured to differ from the design values by < 0.2◦
we currently do not have an accurate external measurement of
the overall polarization angle. We therefore apply a rotation
of ∼ 1◦ to the final Q/U maps to minimize the T B and EB
power (Keating et al. 2013; Kaufman et al. 2013). We empha-
size that this has a negligible effect on the BB bandpowers at
< 200.