Instanton-induced suppression of the singlet axial charge of the proton

We show that instanton-anti-instanton pairs generate an anomalous contribution to the matrix element of the isosinglet axial current in a polarized proton. We compute this contribution for a polarized quark propagating in an instanton vacuum, and we show that for N₁ = 1 it exactly cancels the axial charge of the quark. We show that the anomalous contribution is always anticorrelated to the canonical contribution from current quark polarization, regardless of the details of the quark wave function and instanton density. This may explain the experimentally observed suppression of the first moment of the polarized structure function g₁.