Why is the QCD vacuum important in non-abelian gauge theories?

[blechman]

And if I want the symmetry with charge B+L, I simply apply both of them. Is that correct?

yup!

As for the results of applying that symmetry (the change in the weak $$\theta$$ term), I think I have understood it :)

Thanks a million for that!

As to that reference: that's just filling in all the details by doing the instanton calculation carefully, finding the relevant "zero modes" of the fermions, etc. I skipped all those details for the sake of sanity!

As to ansgar's presentation: thanks for sharing. It looks like a very interesting talk.