The Seventh International Symposium on Chiral Symmetry in Hadrons and Nuclei

The Proton Spin Problem; Measurements of Octet Spin Fractions from Lattice QCD

Not scheduled

20m

BeiHang Univ. Beijing,China

Parellel A

Speaker

Ms Phiala Shanahan (The University of Adelaide)

Description

The 1987 discovery by the European Muon Collaboration, that only a small fraction of the spin of the proton may be attributed to the spin of its three constituent quarks, sparked decades of careful experimental investigation of the phenomenon. Despite extensive efforts, there is no consensus that the so-called `proton spin crisis’ has been adequately resolved. The fraction of the spin of the proton carried by its quarks currently stands at 33 ± 3 ± 5%, if one relies on SU(3) symmetry for the octet axial charge. This is a dramatic suppression with respect to the value of 100% expected in a naive quark model, or even the 65% expected in a relativistic quark model. A number of possible theoretical explanations have been offered for the observed suppression, ranging from a key role for the axial anomaly, to the effect of gluon exchange currents, chiral symmetry and, in the light of insights gained from lattice QCD studies, a combination of both of these effects. It is clearly of great use to be able to place some constraint on candidate explanations of this remarkable phenomenon. We discuss recent work based on lattice QCD simulations from the QCDSF/UKQCD Collaboration. The quark spin content of the octet baryons extracted from the lattice results displays striking variation across the baryon octet. Within the relatively large uncertainties, this observation is supported by the predictions of a model that incorporates the relativistic motion of valence quarks, the pion cloud required by chiral symmetry and an exchange current contribution associated with the one-gluon-exchange hyperfine interaction. The results do not support the hypothesis that the spin suppression observed for the proton is a universal property. It is therefore of considerable interest to investigate further the predictions of models in which the suppression of the spin carried by quarks is dependent on baryon structure.