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The Spin of the proton in chiral effective field theory

Hongna Li; P. Wang; D. B. Leinweber; A. W. ThomasSubjects: Physics >> Nuclear Physics

Proton spin is investigated in chiral effective field theory through an examination of the singlet axial charge,?a0, and the two non-singlet axial charges,?a3?and?a8. Finite-range regularization is considered as it provides an effective model for estimating the role of disconnected sea-quark loop contributions to baryon observables. Baryon octet and decuplet intermediate states are included to enrich the spin and flavour structure of the nucleon, redistributing spin under the constraints of chiral symmetry. In this context, the proton spin puzzle is well understood with the calculation describing all three of the axial charges reasonably well. The strange quark contribution to the proton spin is negative with magnitude 0.01. With appropriate?Q2?evolution, we find the singlet axial charge at the experimental scale to be?a^0=0.31+0.04?0.05, consistent with the range of current experimental values. |

Pure sea-quark contributions to the magnetic form factors of Σ baryons

P. Wang; D. B. Leinweber; A. W. ThomasSubjects: Physics >> Nuclear Physics

We propose the pure sea-quark contributions to the magnetic form factors of?Σ?baryons,?GuΣ??and?GdΣ+, as priority observables for the examination of sea-quark contributions to baryon structure, both in present lattice QCD simulations and possible future experimental measurement.?GuΣ?, the?u-quark contribution to the magnetic form factor of?Σ?, and?GdΣ+, the?d-quark contribution to the magnetic form factor of?Σ+, are similar to the strange quark contribution to the magnetic form factor of the nucleon, but promise to be larger by an order of magnitude. We explore the size of this quantity within chiral effective field theory, including both octet and decuplet intermediate states. The finite range regularization approach is applied to deal with ultraviolet divergences. Drawing on an established connection between quenched and full QCD, this approach makes it possible to predict the sea quark contribution to the magnetic form factor purely from the meson loop. In the familiar convention where the quark charge is set to unity?GuΣ?=GdΣ+. We find a value of??0.38+0.16?0.17?μN, which is about seven times larger than the strange magnetic moment of the nucleon found in the same approach. Including quark charge factors, the?u-quark contribution to the?Σ??magnetic moment exceeds the strange quark contribution to the nucleon magnetic moment by a factor of 14. |

Constraints on $s-\bar s$ asymmetry of the proton in chiral effective theory

X. G. Wang; Chueng-Ryong Ji W. Melnitchouk; Y. Salamu; A. W. Thomas P. WangSubjects: Physics >> Nuclear Physics

We compute the?s?s??asymmetry in the proton in chiral effective theory, using phenomenological constraints based upon existing data. Unlike previous meson cloud model calculations, which accounted for kaon loop contributions with on-shell intermediate states alone, this work includes off-shell terms and contact interactions, which impact the shape of the?s?s??difference. Using a regularization procedure that preserves chiral symmetry and Lorentz invariance, we find that existing data limit the integrated value of the first moment of the asymmetry to the range??0.07×10?3≤?x(s?s?)?≤1.12×10?3?at a scale of?Q2=1GeV2. In contrast to some suggestions in the literature, the magnitude of this correction is too small to account for the NuTeV anomaly. |

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