Many of the distinctive and subtle features of the dynamics in the UA(1)
channel in QCD can be related to gluon topology, more precisely to the
topological susceptibility χ(k2)=i∫d4xeikx,
where Q = {\a_s\over8\pi} {\rm tr} G_{\m\n} \tilde G^{\m\n} is the gluon
topological charge density. The link is the UA(1) axial (ABJ) anomaly. In
this lecture, we describe the anomalous UA(1) chiral Ward identities in a
functional formalism and show how two apparently unrelated `UA(1) problems'
-- the mass of the η′ and the violation of the Ellis-Jaffe sum rule in
polarised deep-inelastic scattering -- can be explained in terms of the gluon
topological susceptibility. They are related through a UA(1) extension of
the Goldberger-Treiman formula, which is derived here for QCD with both
massless and massive quarks.Comment: Lecture at 1998 Zuoz Summer School, `Hidden Symmetries and Higgs
Phenomena'. 22 pages, plain TeX, 2 ps or eps figure