7,525 research outputs found

    Staggered fermion matrix elements using smeared operators

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    We investigate the use of two kinds of staggered fermion operators, smeared and unsmeared. The smeared operators extend over a 444^4 hypercube, and tend to have smaller perturbative corrections than the corresponding unsmeared operators. We use these operators to calculate kaon weak matrix elements on quenched ensembles at β=6.0\beta=6.0, 6.2 and 6.4. Extrapolating to the continuum limit, we find BK(NDR,2GeV)=0.62±0.02(stat)±0.02(syst)B_K(NDR, 2 GeV)= 0.62\pm 0.02(stat)\pm 0.02(syst). The systematic error is dominated by the uncertainty in the matching between lattice and continuum operators due to the truncation of perturbation theory at one-loop. We do not include any estimate of the errors due to quenching or to the use of degenerate ss and dd quarks. For the ΔI=3/2\Delta I = {3/2} electromagnetic penguin operators we find B7(3/2)=0.62±0.03±0.06B_7^{(3/2)} = 0.62\pm 0.03\pm 0.06 and B8(3/2)=0.77±0.04±0.04B_8^{(3/2)} = 0.77\pm 0.04\pm 0.04. We also use the ratio of unsmeared to smeared operators to make a partially non-perturbative estimate of the renormalization of the quark mass for staggered fermions. We find that tadpole improved perturbation theory works well if the coupling is chosen to be \alpha_\MSbar(q^*=1/a).Comment: 22 pages, 1 figure, uses eps

    Physical Results from Partially Quenched Simulation

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    We describe how one can use chiral perturbation theory to obtain results for physical quantities, such as quark masses, using partially quenched simulations.Comment: Written version of two talks at DPF 2000. 6 pages, 2 figure

    Enhanced chiral logarithms in partially quenched QCD

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    I discuss the properties of pions in ``partially quenched'' theories, i.e. those in which the valence and sea quark masses, mVm_V and mSm_S, are different. I point out that for lattice fermions which retain some chiral symmetry on the lattice, e.g. staggered fermions, the leading order prediction of the chiral expansion is that the mass of the pion depends only on mVm_V, and is independent of mSm_S. This surprising result is shown to receive corrections from loop effects which are of relative size mSlnmVm_S \ln m_V, and which thus diverge when the valence quark mass vanishes. Using partially quenched chiral perturbation theory, I calculate the full one-loop correction to the mass and decay constant of pions composed of two non-degenerate quarks, and suggest various combinations for which the prediction is independent of the unknown coefficients of the analytic terms in the chiral Lagrangian. These results can also be tested with Wilson fermions if one uses a non-perturbative definition of the quark mass.Comment: 14 pages, 3 figures, uses psfig. Typos in eqs (18)-(20) corrected (alpha_4 is replaced by alpha_4/2

    Spectra of D-branes with Higgs vevs

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    In this paper we continue previous work on counting open string states between D-branes by considering open strings between D-branes with nonzero Higgs vevs, and in particular, nilpotent Higgs vevs, as arise, for example, when studying D-branes in orbifolds. Ordinarily Higgs vevs can be interpreted as moving the D-brane, but nilpotent Higgs vevs have zero eigenvalues, and so their interpretation is more interesting -- for example, they often correspond to nonreduced schemes, which furnishes an important link in understanding old results relating classical D-brane moduli spaces in orbifolds to Hilbert schemes, resolutions of quotient spaces, and the McKay correspondence. We give a sheaf-theoretic description of D-branes with Higgs vevs, including nilpotent Higgs vevs, and check that description by noting that Ext groups between the sheaves modelling the D-branes, do in fact correctly count open string states. In particular, our analysis expands the types of sheaves which admit on-shell physical interpretations, which is an important step for making derived categories useful for physics.Comment: 46 pages, LaTeX; v2: typos fixed; v3: more typos fixe

    Comforting sentences from the warming room at Inchcolm abbey

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    A fragmentary inscription from the fifteenth century is reconstructed and its source identified, offering an insight into the use of one proverbial source of morally and spiritually encouraging sentences, and opening another little window on to the books available to the canons of Inchcolm

    Applications of Partially Quenched Chiral Perturbation Theory

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    Partially quenched theories are theories in which the valence- and sea-quark masses are different. In this paper we calculate the nonanalytic one-loop corrections of some physical quantities: the chiral condensate, weak decay constants, Goldstone boson masses, B_K and the K+ to pi+ pi0 decay amplitude, using partially quenched chiral perturbation theory. Our results for weak decay constants and masses agree with, and generalize, results of previous work by Sharpe. We compare B_K and the K+ decay amplitude with their real-world values in some examples. For the latter quantity, two other systematic effects that plague lattice computations, namely, finite-volume effects and unphysical values of the quark masses and pion external momenta are also considered. We find that typical one-loop corrections can be substantial.Comment: 22 pages, TeX, refs. added, minor other changes, version to appear in Phys. Rev.

    Observations on discretization errors in twisted-mass lattice QCD

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    I make a number of observations concerning discretization errors in twisted-mass lattice QCD that can be deduced by applying chiral perturbation theory including lattice artifacts. (1) The line along which the PCAC quark mass vanishes in the twisted mass-twisted mass plane makes an angle to the untwisted mass axis which is a direct measure of O(a) terms in the chiral Lagrangian, and is found numerically to be large; (2) Numerical results for pionic quantities in the mass plane show the qualitative properties predicted by chiral perturbation theory, in particular an asymmetry in slopes between positive and negative untwisted quark masses; (3) By extending the description of the ``Aoki regime'' (where m_q is of size a^2 Lambda_QCD^3) to next-to-leading order in chiral perturbation theory I show how the phase transition lines and lines of maximal twist (using different definitions) extend into this region, and give predictions for the functional form of pionic quantities; (4) I argue that the recent claim that lattice artifacts at maximal twist have apparent infrared singularities in the chiral limit results from expanding about the incorrect vacuum state. Shifting to the correct vacuum (as can be done using chiral perturbation theory) the apparent singularities are summed into non-singular, and furthermore predicted, forms. I further argue that there is no breakdown in the Symanzik expansion in powers of lattice spacing, and no barrier to simulating at maximal twist in the Aoki regime.Comment: 20 pages, 6 figures. Published version. More typos corrected, and summary paragraph added to sections II and I

    Current Physics Results from Staggered Chiral Perturbation Theory

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    We review several results that have been obtained using lattice QCD with the staggered quark formulation. Our focus is on the quantities that have been calculated numerically with low statistical errors and have been extrapolated to the physical quark mass limit and continuum limit using staggered chiral perturbation theory. We limit our discussion to a brief introduction to staggered quarks, and applications of staggered chiral perturbation theory to the pion mass, decay constant, and heavy-light meson decay constants.Comment: 18 pages, 4 figures, commissioned review article, to appear in Mod. Phys. Lett.

    Chiral corrections to the axial charges of the octet baryons from quenched QCD

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    We calculate one-loop correction to the axial charges of the octet baryons using quenched chiral perturbation theory, in order to understand chiral behavior of the axial charges in quenched approximation to quantum chromodynamics (QCD). In contrast to regular behavior of the full QCD chiral perturbation theory result, c0+cl2mπ2lnmπ2+c_0+c_{l2}m_\pi^2\,\ln{m_\pi^2}+\cdots, we find that the quenched chiral perturbation theory result, c0Q+(cl0Q+cl2Qmπ2)lnmπ2+c2Qmπ2+c_0^Q+(c_{l0}^Q+c_{l2}^Qm_\pi^2)\ln{m_\pi^2}+c_2^Q m_\pi^2+\cdots, is singular in the chiral limit.Comment: standard LaTeX, 16 pages, 4 epsf figure
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