107 research outputs found

    N Delta - Transition Form Factors at Low Momentum Transfer

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    The three complex form factors entering the ΔNγ\Delta\to N\gamma^\ast vertex are calculated to O(ϵ3){\cal O}(\epsilon^3) in the framework of a chiral effective theory with explicit Δ\Delta(1232) degrees of freedom. Furthermore, the role of presently unknown low energy constants that affect the values of EMR and CMR is elucidated.Comment: 5 pages, 3 figures, Oral contribution given at the 8th International Conference on the Structure of Baryons (Baryons '98), Bonn, Germany, Sept. 22-26, 199

    Scaling and confinement aspects of tadpole improved SU(2) lattice gauge theory and its abelian projection

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    Using a tadpole improved SU(2) gluodynamics action, the nonabelian potential and the abelian potential after the abelian projection are computed. Rotational invariance is found restored at coarse lattices both in the nonabelian theory and in the effective abelian theory resulting from maximal abelian projection. Asymptotic scaling is tested for the SU(2) string tension. Deviation of the order of 66% is found, for lattice spacings between 0.27 and 0.06 fm. Evidence for asymptotic scaling and scaling of the monopole density in maximal abelian projection is also seen, but not at coarse lattices. The scaling behavior is compared with analyses of Wilson action results, using bare and renormalized coupling schemes. Using extended monopoles, evidence is found that the gauge dependence of the abelian projection reflects short distance fluctuations, and may thus disappear at large scales.Comment: 28 pages, RevTeX, 12 figures using epsfig (included); accepted for publication in Physical Revie

    Magnetization process of the spin-1/2 XXZ models on square and cubic lattices

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    The magnetization process of the spin-1/2 antiferromagnetic XXZ model with Ising-like anisotropy in the ground state is investigated. We show numerically that the Ising-like XXZ models on square and cubic lattices show a first-order phase transition at some critical magnetic field. We estimate the value of the critical field and the magnetization jump on the basis of the Maxwell construction. The magnetization jump in the Ising-limit is investigated by means of perturbation theory. Based on our numerical results, we briefly discuss the phase diagram of the extended Bose-Hubbard model in the hard-core limit.Comment: 13 pages, RevTex, 7 PostScript figures, to appear in Phys.Rev.

    Test of the QCD vacuum with the sources in higher representations

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    Recent accurate measurement by G.Bali of static potentials between sources in various SU(3) representations provides a crucial test of the QCD vacuum and of different theoretical approaches to the confinement. In particular, the Casimir scaling of static potentials found for all measured distances implies a strong suppression of higher cumulants and a high accuracy of the Gaussian stochastic vacuum. Most popular models are in conflict with these measurements.Comment: LaTeX, 7 page

    String breaking by dynamical fermions in three-dimensional lattice QCD

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    The first observation is made of hadronic string breaking due to dynamical fermions in zero temperature lattice QCD. The simulations are done for SU(2) color in three dimensions, with two flavors of staggered fermions. The results have clear implications for the large scale simulations that are being done to search (so far, without success) for string breaking in four-dimensional QCD. In particular, string breaking is readily observed using only Wilson loops to excite a static quark-antiquark pair. Improved actions on coarse lattices are used, providing an extremely efficient means to access the quark separations and propagation times at which string breaking occurs.Comment: Revised version to appear in Physical Review D, has additional discussion of the results, additional references, modified title, larger figure

    Static SU(3) potentials for sources in various representations

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    The potentials and string tensions between static sources in a variety of representations (fundamental, 8, 6, 15-antisymmetric, 10, 27 and 15-symmetric) have been computed by measuring Wilson loops in pure gauge SU(3). The simulations have been done primarily on anisotropic lattices, using a tadpole improved action improved to O(a_{s}^4). A range of lattice spacings (0.43 fm, 0.25 fm and 0.11 fm) and volumes (83×248^3\times 24, 103×2410^3 \times 24, 163×2416^3 \times 24 and 183×2418^3 \times 24) has been used in an attempt to control discretization and finite volume effects. At intermediate distances, the results show approximate Casimir scaling. Finite lattice spacing effects dominate systematic error, and are particularly large for the representations with the largest string tensions.Comment: Version to appear in PR

    Casimir Scaling from Center Vortices: Towards an Understanding of the Adjoint String Tension

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    We argue that the approximate ``Casimir scaling'' of the string tensions of higher-representation Wilson loops is an effect due to the finite thickness of center vortex configurations. It is shown, in the context of a simple model of the Z(2) vortex core, how vortex condensation in Yang-Mills theory can account for both Casimir scaling in intermediate size loops, and color-screening in larger loops. An implication of our model is that the deviations from exact Casimir scaling, which tend to grow with loop size, become much more pronounced as the dimensionality of the group representation increases.Comment: 13 pages, including 3 eps figures, Latex2e. Two references adde

    Tadpole-improved SU(2) lattice gauge theory

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    A comprehensive analysis of tadpole-improved SU(2) lattice gauge theory is made. Simulations are done on isotropic and anisotropic lattices, with and without improvement. Two tadpole renormalization schemes are employed, one using average plaquettes, the other using mean links in Landau gauge. Simulations are done with spatial lattice spacings asa_s in the range of about 0.1--0.4 fm. Results are presented for the static quark potential, the renormalized lattice anisotropy at/asa_t/a_s (where ata_t is the ``temporal'' lattice spacing), and for the scalar and tensor glueball masses. Tadpole improvement significantly reduces discretization errors in the static quark potential and in the scalar glueball mass, and results in very little renormalization of the bare anisotropy that is input to the action. We also find that tadpole improvement using mean links in Landau gauge results in smaller discretization errors in the scalar glueball mass (as well as in the static quark potential), compared to when average plaquettes are used. The possibility is also raised that further improvement in the scalar glueball mass may result when the coefficients of the operators which correct for discretization errors in the action are computed beyond tree level.Comment: 14 pages, 7 figures (minor changes to overall scales in Fig.1; typos removed from Eqs. (3),(4),(15); some rewording of Introduction

    Weak gauge principle and electric charge quantization

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    Starting from a weak gauge principle we give a new and critical revision of the argument leading to charge quantization on arbitrary spacetimes. The main differences of our approach with respect to previous works appear on spacetimes with non trivial torsion elements on its second integral cohomology group. We show that in these spacetimes there can be topologically non-trivial configurations of charged fields which do not imply charge quantization. However, the existence of a non-exact electromagnetic field always implies the quantization of charges. Another consequence of the theory for spacetimes with torsion is the fact that it gives rise to two natural quantization units that could be identified with the electric quantization unit (realized inside the quarks) and with the electron charge. In this framework the color charge can have a topological origin, with the number of colors being related to the order of the torsion subgroup. Finally, we discuss the possibility that the quantization of charge may be due to a weak non-exact component of the electromagnetic field extended over cosmological scales.Comment: Latex2e, 24 pages, no figure

    Adjoint "quarks" on coarse anisotropic lattices: Implications for string breaking in full QCD

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    A detailed study is made of four dimensional SU(2) gauge theory with static adjoint ``quarks'' in the context of string breaking. A tadpole-improved action is used to do simulations on lattices with coarse spatial spacings asa_s, allowing the static potential to be probed at large separations at a dramatically reduced computational cost. Highly anisotropic lattices are used, with fine temporal spacings ata_t, in order to assess the behavior of the time-dependent effective potentials. The lattice spacings are determined from the potentials for quarks in the fundamental representation. Simulations of the Wilson loop in the adjoint representation are done, and the energies of magnetic and electric ``gluelumps'' (adjoint quark-gluon bound states) are calculated, which set the energy scale for string breaking. Correlators of gauge-fixed static quark propagators, without a connecting string of spatial links, are analyzed. Correlation functions of gluelump pairs are also considered; similar correlators have recently been proposed for observing string breaking in full QCD and other models. A thorough discussion of the relevance of Wilson loops over other operators for studies of string breaking is presented, using the simulation results presented here to support a number of new arguments.Comment: 22 pages, 14 figure
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