Target independence of the `proton spin' effect

Abstract

Recent work by the author in collaboration with S. Narison and G. Veneziano on the EMC-SMC-SLAC `proton spin' effect is reviewed. This uses a novel approach to deep inelastic scattering in which the matrix elements arising from the OPE are factorised into composite operator propagators and proper vertices. For polarised $\mu p$ scattering, the composite operator propagator is equated to the square root of the first moment of the QCD topological susceptibility, $\sqrt{{\chi }^{\prime }(0)}$. We evaluate ${\chi }^{\prime }(0)$ using QCD spectral sum rules and find a significant suppression relative to its OZI expectation. This is identified as the source of the violation of the Ellis-Jaffe sum rule for the first moment of the polarised proton structure function $g_1^p$. Our predictions, ${\int }_0^{1}dx{g}_1^p(x;Q^2=10Ge{V}^2)=0.143\pm 0.005$ and $\Delta \Sigma =0.353\pm 0.052$, are in excellent agreement with the new SMC data. This supports our earlier conjecture that the suppression in the flavour singlet component of the first moment of $g_1^p$ is a target-independent feature of QCD related to the $U(1)$ anomaly and is not a special property of the proton structure.

Comments: 6 pages, SWAT 94-48