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The Proton spin crisis: Another ABJ anomaly?
From the Planck length to the Hubble radius. Proceedings, Erice 1998, Pages: 79 - 105
Swansea University Author: Graham Shore
For a decade, the ‘proton spin’ problem – the anomalous suppression observed in the flavour singlet component of the first moment of the polarised proton structure function g1p(x; Q2) – has puzzled and intrigued theorists and experimentalists alike. The consequent research effort has indeed been imp...
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For a decade, the ‘proton spin’ problem – the anomalous suppression observed in the flavour singlet component of the first moment of the polarised proton structure function g1p(x; Q2) – has puzzled and intrigued theorists and experimentalists alike. The consequent research effort has indeed been impressive: to date, the original EMC paper alone has nearly one thousand citations.In this lecture, we first give a brief review of the ‘proton spin’ problem from the standard viewpoint of the parton model. We explain why the 1988 observation by the EMC of a violation of the Ellis-Jaffe sum rule for g1p was initially mis-interpreted in terms of quark spins and how the problem is resolved in the context of the full QCD parton model.We then describe an alternative, complementary approach to the description of deep inelastic scattering (DIS), the ‘CPV’ method, which allows the problem to be viewed in a new light. From this perspective, the Ellis-Jaffe sum rule is simply seen to be equivalent to the OZI approximation for the forward proton matrix element of the flavour singlet axial current. The ‘proton spin’ problem is therefore one of understanding the origin of the OZI violation observed in this channel. As such, it is one more addition to the collection of ‘UA(1) problems’ in QCD – phenomena whose interpretation depends on the presence of the ABJ axial anomaly and the implicit relation with gluon topology. As we shall show, the observed suppression in the first moment of g1p is due to topological charge screening by the QCD vacuum itself, and a quantitative resolution in terms of an anomalous suppression of the slope of the gluon topological susceptibility is proposed.An immediate consequence of this explanation is that the suppression in g1p is in fact a target independent phenomenon, which would in principle be true for polarised DIS on any hadronic target. Not only is the ‘proton spin’ problem nothing to do with spin – it is not even a special property of the proton! To test this idea, we have proposed a set of semi-inclusive polarised DIS experiments, which could be performed at e.g. polarised HERA, and which would provide independent confirmation of the mechanism of topological charge screening by the QCD vacuum.
Faculty of Science and Engineering