分类： 物理学 >> 核物理学 提交时间： 2016-09-18
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： The structure and decay properties of d∗ have been detailedly investigated in both the chiral SU(3) quark model and the extended chiral SU(3) quark model that describe the energies of baryon ground states and the nucleon-nucleon (NN) scattering data satisfactorily. By performing a dynamical coupled-channels study of the system of ΔΔ and hidden-color channel (CC) with quantum numbers I(JP)=0(3+) in the framework of the resonating group method (RGM), we find that the d∗ has a mass of about 2.38−2.42 GeV and a root-mean-square radius (RMS) of about 0.76−0.88 fm. The channel wave function is extracted by a projection of the RGM wave function onto the physical basis, and the fraction of CC component in the d∗ is found to be about 66%−68%, which indicates that the d∗ is a hexaquark-dominated exotic state. Based on this scenario the partial decay widths of d∗→dπ0π0 and d∗→dπ+π− are further explicitly evaluated and the total width is then obtained by use of the branching ratios extracted from the measured cross sections of other possible decay channels. Both the mass and the decay width of d∗ calculated in this work are compatible with the data (M≈2380 MeV, Γ≈70 MeV) reported by WASA-at-COSY Collaboration.
分类： 物理学 >> 核物理学 提交时间： 2016-09-13
摘要： 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.
分类： 物理学 >> 核物理学 提交时间： 2016-08-31
摘要： 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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