78,414 research outputs found

    Interplay of Spin and Orbital Angular Momentum in the Proton

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    We derive the consequences of the Myhrer-Thomas explanation of the proton spin problem for the distribution of orbital angular momentum on the valence and sea quarks. After QCD evolution these results are found to be in very good agreement with both recent lattice QCD calculations and the experimental constraints from Hermes and JLab

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

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    We propose the pure sea-quark contributions to the magnetic form factors of Σ\Sigma baryons, GΣ−uG_{\Sigma^-}^u and GΣ+dG_{\Sigma^+}^d, as priority observables for the examination of sea-quark contributions to baryon structure, both in present lattice QCD simulations and possible future experimental measurement. GΣ−uG_{\Sigma^-}^u, the uu-quark contribution to the magnetic form factor of Σ−\Sigma^-, and GΣ+dG_{\Sigma^+}^d, the dd-quark contribution to the magnetic form factor of Σ+\Sigma^+, 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 GΣ−u=GΣ+dG_{\Sigma^-}^u = G_{\Sigma^+}^d. We find a value of −0.38−0.17+0.16 μN-0.38^{+0.16}_{-0.17}\ \mu_N, which is about seven times larger than the strange magnetic moment of the nucleon found in the same approach. Including quark charge factors, the uu-quark contribution to the Σ−\Sigma^- magnetic moment exceeds the strange quark contribution to the nucleon magnetic moment by a factor of 14.Comment: 5 pages, 3 figures. arXiv admin note: text overlap with arXiv:1312.337

    Progress in resolving charge symmetry violation in nucleon structure

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    Recent work unambiguously resolves the level of charge symmetry violation in moments of parton distributions using 2+1-flavor lattice QCD. We introduce the methods used for that analysis by applying them to determine the strong contribution to the proton-neutron mass difference. We also summarize related work which reveals that the fraction of baryon spin which is carried by the quarks is in fact structure-dependent rather than universal across the baryon octet.Comment: 8 pages, 4 figures; presented at "The Seventh International Symposium on Chiral Symmetry in Hadrons and Nuclei", BeiHang Univ. Beijing, Chin

    Updated Analysis of the Mass of the H Dibaryon from Lattice QCD

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    Recent lattice QCD calculations from the HAL and NPLQCD Collaborations have reported evidence for the existence of a bound state with strangeness -2 and baryon number 2 at quark masses somewhat higher than the physical values. A controlled chiral extrapolation of these lattice results to the physical point suggested that the state, identified with the famed H dibaryon, is most likely slightly unbound (by 13 ±\pm 14 MeV) with respect to the Λ−−Λ\Lambda--\Lambda threshold. We report the results of an updated analysis which finds the H unbound by 26 ±\pm 11 MeV. Apart from the insight it would give us into how QCD is realized in Nature, the H is of great interest because of its potential implications for the equation of state of dense matter and studies of neutron stars. It may also explain the enhancement above the Λ−−Λ\Lambda--\Lambda threshold already reported experimentally. It is clearly of great importance that the latter be pursued in experiments at the new J-PARC facility.Comment: Invited presentation at APPC12 (12th Asia Pacific Physics Conference), July 14-19, 2013, Chiba, Japa

    Decoherence of spin echoes

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    We define a quantity, the so-called purity fidelity, which measures the rate of dynamical irreversibility due to decoherence, observed e.g in echo experiments, in the presence of an arbitrary small perturbation of the total (system + environment) Hamiltonian. We derive a linear response formula for the purity fidelity in terms of integrated time correlation functions of the perturbation. Our relation predicts, similarly to the case of fidelity decay, faster decay of purity fidelity the slower decay of time correlations is. In particular, we find exponential decay in quantum mixing regime and faster, initially quadratic and later typically gaussian decay in the regime of non-ergodic, e.g. integrable quantum dynamics. We illustrate our approach by an analytical calculation and numerical experiments in the Ising spin 1/2 chain kicked with tilted homogeneous magnetic field where part of the chain is interpreted as a system under observation and part as an environment.Comment: 22 pages, 10 figure

    Liquid-gas phase transition in nuclear matter including strangeness

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    We apply the chiral SU(3) quark mean field model to study the properties of strange hadronic matter at finite temperature. The liquid-gas phase transition is studied as a function of the strangeness fraction. The pressure of the system cannot remain constant during the phase transition, since there are two independent conserved charges (baryon and strangeness number). In a range of temperatures around 15 MeV (precise values depending on the model used) the equation of state exhibits multiple bifurcates. The difference in the strangeness fraction fsf_s between the liquid and gas phases is small when they coexist. The critical temperature of strange matter turns out to be a non-trivial function of the strangeness fraction.Comment: 15 pages, 7 figure

    Towards a Connection Between Nuclear Structure and QCD

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    As we search for an ever deeper understanding of the structure of hadronic matter one of the most fundamental questions is whether or not one can make a connection to the underlying theory of the strong interaction, QCD. We build on recent advances in the chiral extrapolation problem linking lattice QCD at relatively large ``light quark'' masses to the physical world to estimate the scalar polarizability of the nucleon. The latter plays a key role in modern relativistic mean-field descriptions of nuclei and nuclear matter (such as QMC) and, in particular, leads to a very natural saturation mechanism. We demonstrate that the value of the scalar polarizability extracted from the lattice data is consistent with that needed for a successful description of nuclei within the framework of QMC. In a very real sense this is the first hint of a direct connection between QCD and the properties of finite nuclei.Comment: Lecture presented at: 18th Nishinomiya-Yukawa Memorial Symposium On Strangeness In Nuclear Matter : 4-5 Dec 2003, Nishinomiya, Japa

    Neutron stars and strange stars in the chiral SU(3) quark mean field model

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    We investigate the equations of state for pure neutron matter and strange hadronic matter in β\beta-equilibrium, including Λ\Lambda, Σ\Sigma and Ξ\Xi hyperons. The masses and radii of pure neutron stars and strange hadronic stars are obtained. For a pure neutron star, the maximum mass is about 1.8Msun1.8 M_{\mathrm{sun}}, while for a strange hadronic star, the maximum mass is around 1.45Msun1.45 M_{\mathrm{sun}}. The typical radii of pure neutron stars and strange hadronic stars are about 11.0-12.3 km and 10.7-11.7 km, respectively.Comment: 18 pages, 7 figure

    Sigma terms from an SU(3) chiral extrapolation

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    We report a new analysis of lattice simulation results for octet baryon masses in 2+1-flavor QCD, with an emphasis on a precise determination of the strangeness nucleon sigma term. A controlled chiral extrapolation of a recent PACS-CS Collaboration data set yields baryon masses which exhibit remarkable agreement both with experimental values at the physical point and with the results of independent lattice QCD simulations at unphysical meson masses. Using the Feynman-Hellmann relation, we evaluate sigma commutators for all octet baryons. The small statistical uncertainty, and considerably smaller model-dependence, allows a signifcantly more precise determination of the pion-nucleon sigma commutator and the strangeness sigma term than hitherto possible, namely {\sigma}{\pi}N=45 \pm 6 MeV and {\sigma}s = 21 \pm 6 MeV at the physical point.Comment: 4 pages, 4 figure
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