801 research outputs found

    Understanding Parton Distributions from Lattice QCD: Present Limitations and Future Promise

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    This talk will explain how ground state matrix elements specifying moments of quark density and spin distributions in the nucleon have been calculated in full QCD, show how physical extrapolation to the chiral limit including the physics of the pion cloud resolves previous apparent conflicts with experiment, and describe the computational resources required for a definitive comparison with experiment.Comment: Proceedings of Ferrara Workshop on the QCD Structure of the Nucleon 10 pages, 6 figure

    Understanding Hadron Structure Using Lattice QCD

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    Numerical evaluation of the path integral for QCD on a discrete space-time lattice has been used to calculate ground state matrix elements specifying moments of quark density and spin distributions. This talk will explain how these matrix elements have been calculated in full QCD using dynamical quarks, show how physical extrapolation to the chiral limit including the physics of the pion cloud resolves previous apparent conflicts with experiment, and describe the computational resources required for a definitive comparison with experiment.Comment: 8 pages, 7 figures, using BoxedEPS and espcrc1 macros; Invited Talk at Third International Conference on Perspectives in Hadronic Physics, Trieste; email correspondenc to [email protected]

    Incommensurate nematic fluctuations in the two-dimensional Hubbard model

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    We analyze effective d-wave interactions in the two-dimensional extended Hubbard model at weak coupling and small to moderate doping. The interactions are computed from a renormalization group flow. Attractive d-wave interactions are generated via antiferromagnetic spin fluctuations in the pairing and charge channels. Above Van Hove filling, the d-wave charge interaction is maximal at incommensurate diagonal wave vectors, corresponding to nematic fluctuations with a diagonal modulation. Below Van Hove filling a modulation along the crystal axes can be favored. The nematic fluctuations are enhanced by the nearest-neighbor interaction in the extended Hubbard model, but they always remain smaller than the dominant antiferromagnetic, pairing, or charge density wave fluctuations.Comment: 8 pages, 4 figures; figures improve

    Insight into Hadron Structure from Lattice QCD

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    A variety of evidence from lattice QCD is presented revealing the dominant role of instantons in the propagation of light quarks in the QCD vacuum and in light hadron structure. The instanton content of lattice gluon configurations is extracted, and observables calculated from the instantons alone are shown to agree well with those calculated using all gluons. The lowest 128 eigenfunctions of the Dirac operator are calculated and shown to exhibit zero modes localized at the instantons. Finally, the zero mode contributions to the quark propagator alone are shown to account for essentially the full strength of the rho and pion resonances in the vector and pseudoscalar correlation functions

    Particle number fluctuations and correlations in transfer reactions obtained using the Balian-V\'en\'eroni variational principle

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    The Balian-V\'en\'eroni (BV) variational principle, which optimizes the evolution of the state according to the relevant observable in a given variational space, is used at the mean-field level to determine the particle number fluctuations in fragments of many-body systems. For fermions, the numerical evaluation of such fluctuations requires the use of a time-dependent Hartree-Fock (TDHF) code. Proton, neutron and total nucleon number fluctuations in fragments produced in collisions of two 40Ca are computed for a large range of angular momenta at a center of mass energy E_cm=128 MeV, well above the fusion barrier. For deep-inelastic collisions, the fluctuations calculated from the BV variational principle are much larger than standard TDHF results, and closer to mass and charge experimental fluctuations. For the first time, correlations between proton and neutron numbers are determined within a quantum microscopic approach. These correlations are shown to be larger with exotic systems where charge equilibration occurs.Comment: Accepted for publication in Phys. Rev. Lett. New version with more detailed comparison with experimental data and prediction for exotic beam

    Interplay between spin density wave and π\pi phase shifted superconductivity in the Fe pnictide superconductors

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    We explore if the phase separation or coexistence of the spin density wave (SDW) and superconductivity (SC) states has any relation to the incommensurability of the SDW in the Fe pnictide superconductors. A systematic method of determining the phase separation or coexistence was employed by computing the anisotropy coefficient β\beta from the the 4th order terms of the Ginzburg--Landau (GL) expansion of the free energy close to the tricritical/tetracritical point. It was complemented by the self-consistent numerical iterations of the gap equations to map out the boundaries between the phase separation and coexistence of the SDW and SC phases, and between commensurate (C) and incommensurate (IC) SDW in the temperature--doping plane. Our principal results for the sign reversed ss-wave pairing SC, in terms of the multicritical temperature, TcT_c, the phase separation/coexistence boundary between the SDW and SC, TT^*, and the boundary between C/IC SDW, TMT_M^*, are: (a) IC-SDW and SC coexist for Tc<TT_c < T^* and phase separate otherwise, (b) SDW takes the C form for Tc>TMT_c>T_M^* and IC form for Tc<TMT_c<T_M^*, and (c) the thermodynamic first order phase transition intervenes in between the C-SDW and IC-SDW boundary for large TM0T_M^0, where TM0T_M^0 is the SDW transition temperature at zero doping, T=0.35 TM0T^*=0.35 ~T_M^0 and TM=0.56 TM0T_M^*=0.56\ T_M^0. The intervention makes the phase diagram more complicated than previously reported. By contrast no coexistence was found for the equal sign pairing SC. These results will be compared with the experimental reports in the Fe pnictide superconductors.Comment: 9 pages, 4 figures, Submitted to Phys.Rev.

    Spontaneous-Symmetry-Breaking Mechanism of Adiabatic Pumping

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    We consider heterostructures consisting of regions with a continuous symmetry in contact with regions wherein the symmetry is spontaneously broken. The low-frequency dynamics of the corresponding order parameter are shown to induce nonequilibrium transport, a ``pumping,'' out of the symmetry-broken regions, which is governed by the generator of the broken-symmetry operator. This pumping damps Goldstone-mode excitations and transfers them beyond traditional (static) proximity length scales. Our general conclusions are discussed for specific examples of order parameters in helimagnets, charge/spin-density waves, superconductors, and ferromagnets. We carry out a detailed calculation of such pumping for spiral magnetic orders in helimagnets possessing a duality in the representation of its symmetry-broken states.Comment: 5 pages, 2 figure

    The eta ' signal from partially quenched Wilson fermions

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    We present new results from our ongoing study of flavor singlet pseudoscalar mesons in QCD. Our approach is based on (a) performing truncated eigenmode expansions for the hairpin diagram and (b) incorporating the ground state contribution for the connected meson propagator. First, we explain how the computations can be substantially improved by even-odd preconditioning. We extend previous results on early mass plateauing in the eta' channel of two-flavor full QCD with degenerate sea and valence quarks to the partially quenched situation. We find that early mass plateau formation persists in the partially quenched situation.Comment: Lattice2002(spectrum), 3 pages, 5 figure
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