Nucleon spin structure, topological susceptibility and the η′ singlet axial vector coupling

The observed small value of the first moment of the polarized nucleon spin structure function g1 may be interpreted, in the Veneziano-Shore approach, as a suppression of the first moment χ′(0) of the QCD topological susceptibility. I give an extension of the Witten-Veneziano argument for the U(1) problem, which yields the O(1N) correction to the N = ∞ relation χ′(0)F02 = 1 (where F0 is the η′ axial vector coupling). The correction, although negative, seems too small to account for the data. I further argue that the (η, η′) → γγ and Jψ → (η, η′)γ decays indicate an enhancement rather than a suppression of F0. A substantial gluon-like contribution in 〈0|ϖμjμ5(0)|γγ〉q2=0, whi parallel a similar one in the corresponding nucleon matrix element, is suggested.