13,957 research outputs found

    Taste symmetry breaking with HYP-smeared staggered fermions

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    We study the impact of hypercubic (HYP) smearing on the size of taste breaking for staggered fermions, comparing to unimproved and to asqtad-improved staggered fermions. As in previous studies, we find a substantial reduction in taste-breaking compared to unimproved staggered fermions (by a factor of 4-7 on lattices with spacing a0.1a\approx 0.1 fm). In addition, we observe that discretization effects of next-to-leading order in the chiral expansion (O(a2p2){\cal O}(a^2 p^2)) are markedly reduced by HYP smearing. Compared to asqtad valence fermions, we find that taste-breaking in the pion spectrum is reduced by a factor of 2.5-3, down to a level comparable to the expected size of generic O(a2){\cal O}(a^2) effects. Our results suggest that, once one reaches a lattice spacing of a0.09a\approx 0.09 fm, taste-breaking will be small enough after HYP smearing that one can use a modified power counting in which O(a2)O(p2){\cal O}(a^2) \ll {\cal O}(p^2), simplify fitting to phenomenologically interesting quantities.Comment: 14 pages, 13 figures, references updated, minor change

    Four-point functions and kaon decays in a minimal AdS/QCD model

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    We study the predictions of holographic QCD for various observable four-point quark flavour current-current correlators. The dual 5-dimensional bulk theory we consider is a SU(3)L×SU(3)RSU(3)_L \times SU(3)_R Yang Mills theory in a slice of AdS5AdS_5 spacetime with boundaries. Particular UV and IR boundary conditions encode the spontaneous breaking of the dual 4D global chiral symmetry down to the SU(3)VSU(3)_V subgroup. We explain in detail how to calculate the 4D four-point quark flavour current-current correlators using the 5D holographic theory, including interactions. We use these results to investigate predictions of holographic QCD for the ΔI=1/2\Delta I = 1/2 rule for kaon decays and the BKB_K parameter. The results agree well in comparison with experimental data, with an accuracy of 25% or better. The holographic theory automatically includes the contributions of the meson resonances to the four-point correlators. The correlators agree well in the low-momentum and high-momentum limit, in comparison with chiral perturbation theory and perturbative QCD results, respectively.Comment: Published version, title changed to conform with Journal format, references and clarifying remarks added, 40 pages, 5 figure

    BRST symmetry of SU(2) Yang-Mills theory in Cho--Faddeev--Niemi decomposition

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    We determine the nilpotent BRST and anti-BRST transformations for the Cho--Faddeev-Niemi variables for the SU(2) Yang-Mills theory based on the new interpretation given in the previous paper of the Cho--Faddeev-Niemi decomposition. This gives a firm ground for performing the BRST quantization of the Yang--Mills theory written in terms of the Cho--Faddeev-Niemi variables. We propose also a modified version of the new Maximal Abelian gauge which could play an important role in the reduction to the original Yang-Mills theory.Comment: 11 pages, no figure; Introduction improved, 3 references adde

    High Energy Physics from High Performance Computing

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    We discuss Quantum Chromodynamics calculations using the lattice regulator. The theory of the strong force is a cornerstone of the Standard Model of particle physics. We present USQCD collaboration results obtained on Argonne National Lab's Intrepid supercomputer that deepen our understanding of these fundamental theories of Nature and provide critical support to frontier particle physics experiments and phenomenology.Comment: Proceedings of invited plenary talk given at SciDAC 2009, San Diego, June 14-18, 2009, on behalf of the USQCD collaboratio

    Collective resonance modes of Josephson vortices in sandwiched stack of Bi2_{2}Sr2_{2}CaCu2_{2}O8+x_{8+x} intrinsic Josephson junctions

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    We observed splitting of the low-bias vortex-flow branch in a dense-Josephson-vortex state into multiple sub-branches in current-voltage characteristics of intrinsic Josephson junctions (IJJs) of Bi2_{2}Sr2_{2}CaCu2_{2}O8+x_{8+x} single crystals in the long-junction limit. Each sub-branch corresponds to a plasma mode in serially coupled Josephson junctions. Splitting into low-bias linear sub-branches with a spread in the slopes and the inter-sub-branch mode-switching character are in good quantitative agreement with the prediction of the weak but finite inter-junction capacitive-coupling model incorporated with the inductive coupling. This suggests the importance of the role of the capacitive coupling in accurately describing the vortex dynamics in serially stacked IJJs.Comment: 4 pages, 3 figures, 1 tabl
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