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We report on a high statistics lattice QCD calculation of the mass of the eta and eta' mesons using ASQTAD improved staggered fermions. The calculation used two ensembles with different lattice spacings and pion masses. We also report results for eta-eta' mixing. The results are in satisfactory agreement with other lattice calculations using other fermion formulations and with experiment, given the the unphysical quark masses used. We see no evidence of abnormal behaviour at the lattice spacings studied., Comment: 10 pages, 10 figures; replacing with published version (1 figure and some discussion added; typos corrected)

We present the first 2+1 flavour spectrum measurements of glueball states using high statistics simulations with improved staggered fermions. We find a spectrum consistent with quenched measurements of scalar, pseudoscalar andtensor glueball states. The measurements were made using 5000 configurations at a lattice spacing of 0.123 fm and pion mass of 280 MeV and 3000 configurations at 0.092 fm with a pion mass of 360 MeV. We see some evidence of coupling to 2 pion states. We compare our results with the experimental glueball candidate spectrum as well as quenched glueball estimates., Comment: 22 pages, 19 figures and 8 tables, minor additions on mixing post-referee

Accurately calculating the mass of flavor-singlet meson states from numerical lattice simulations is an important milestone for lattice QCD. Careful measurement of the full pseudoscalar flavor-singlet propagator is also a crucial step in understanding the dynamics of the fermion sea on the lattice, in particular for potentially non-trivial formulations such as with 2+1-flavor staggered fermions. We briefly describe details of a dynamical QCD calculation using improved staggered fermions, with 30,000 trajectories, that was run for our studies of flavour singlet mesons., Comment: 7 pages, 5 figures. Presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, Germany. (hadron spectroscopy)

We present the first 2+1 flavour lattice QCD calculations of pseudoscalar flavour-singlet propagators using improved staggered fermions. We explore the relevant techniques and discuss prospects for the larger scale studies now in progress. The disconnected correlator is shown to have a highly non-Gaussian distribution and reliable estimates of the errors require care., Comment: 44 pages,24 Figures

The Asqtad improved staggered fermion formalism has been a valuable tool in successfully calculating the non-singlet parts of the hadronic spectrum. We are engaged in a project to calculate the spectrum of the pseudoscalar singlet mesons with 2+1-flavor Asqtad staggered gauge configurations. Propagators of flavor-singlet states incorporate contributions from both disconnected and connected diagrams, and hence are sensitive to any differences in the actions governing the sea and valence fermions on the lattice. As such, they also present the possibility of a probe of the validity of the ``fourth-root trick'' in the staggered fermion formulation. We present an update on our progress toward measuring the $\eta'$ mass on 2+1-flavor Asqtad staggered gauge configurations, including a review of methods and preliminary results. We also show a strong correlation between ${\rm Tr}(\gamma_5\otimes 1)$ and the topological charge in these configurations, as predicted by the index theorem., Comment: 7 pages, 5 figures, talk presented at Lattice2006 (Hadron Spectroscopy)

We look for the effect of open decay channels on the masses of baryons in unquenched lattice QCD. We apply variational smearing using fuzzed basis states to a staggered nucleon operator. The signal for the s_bar s current in the nucleon is studied. The lattice calculations are done using improved staggered fermions., Comment: 6 pages, 1 figure. Presented at 23rd International Symposium on Lattice Field Field: Lattice 2005, Trinity College, Dublin, Ireland, 25-30 Jul 2005

Because the propagators of flavor-singlet states incorporate disconnected diagrams, they are uniquely sensitive to any differences in the actions governing sea and valence fermions on the lattice. As such, they present an important test of the validity of the ``fourth-root trick'' in the staggered fermion formulation. The pseudoscalar's relationship to topological charge also makes it of theoretical interest. We present preliminary results from our measurements of flavor-singlet pseudoscalar mesons on 2+1 flavor Asqtad lattices, and discuss some strategies for improving the signal-to-error ratio of disconnected singlet correlators., Comment: 7 pages, 5 figures, to appear in proceedings of Computational Hadron Physics, Nicosia, Cyprus, 14-17 Sep 2005

We present results of an exploratory study of singlet scalar states in unquenched QCD using both glueball and meson operators. Results for non-singlet non-strange scalar mesons are also presented. We use Asqtad improved staggered fermions and gauge configurations generated by the MILC collaboration at lattice spacings of .12 and .09 fm. In this formulation, the glueball mass is not significantly different from the quenched value at finite lattice spacing. Significant taste violations are present in the scalar sector. At light quark masses, decay channels complicate the mass determinations. There is some evidence that the non-strange singlet meson lies below the non-singlet meson., Comment: Lattice 2005 (hadron spectrum and quark masses), 6 pages, 4 figures

We report on progress in measuring disconnected correlators associated with pseudoscalar flavor-singlet mesons. This will eventually allow us to compute the masses of the eta and eta' mesons. Flavor-singlet physics also presents an interesting test of the staggered fermion formulation, as disconnected correlators are sensitive to whether the same action governs both sea quarks and valence quarks. It can also help test the validity of the ``fourth-root trick'' used in unquenched lattice calculations where the number of flavors $N_f<4$., Comment: Talk presented at Lattice 2005 (Hadron spectrum and quark masses), 6 pages, 3 figures

We present results for QCD with 2 degenerate flavours of quark using a non-perturbatively improved action on a lattice volume of $16^3\times32$ where the bare gauge coupling and bare dynamical quark mass have been chosen to maintain a fixed physical lattice spacing and volume (1.71 fm). By comparing measurements from these matched ensembles, including quenched ones, we find evidence of dynamical quark effects on the short distance static potential, the scalar glueball mass and the topological susceptibility. There is little evidence of effects on the light hadron spectrum over the range of quark masses studied ($m_{\pi}/m_{\rho}\geq 0.60$)., Comment: Lattice 2000 (Spectrum and quark masses), 4 pages, 5 figures

We investigate instability and reversibility within Hybrid Monte Carlo simulations using a non-perturbatively improved Wilson action. We demonstrate the onset of instability as tolerance parameters and molecular dynamics step sizes are varied. We compare these findings with theoretical expectations and present limits on simulation parameters within which a stable and reversible algorithm is obtained for physically relevant simulations. Results of optimisation experiments with respect to tolerance prarameters are also presented., Comment: RevTeX, Some results here were presented at Vielat 99, Vienna, Austria, Sept 1999 22 Pages, 10 figures, to be submitted to PRD

The Lanczos algorithm for matrix tridiagonalisation suffers from strong numerical instability in finite precision arithmetic when applied to evaluate matrix eigenvalues. The mechanism by which this instability arises is well documented in the literature. A recent application of the Lanczos algorithm proposed by Bai, Fahey and Golub allows quadrature evaluation of inner products of the form $\psi^\dagger g(A) \psi$. We show that this quadrature evaluation is numerically stable and explain how the numerical errors which are such a fundamental element of the finite precision Lanczos tridiagonalisation procedure are automatically and exactly compensated in the Bai, Fahey and Golub algorithm. In the process, we shed new light on the mechanism by which roundoff error corrupts the Lanczos procedure, Comment: 3 pages, Lattice 99 contribution

We investigate a number of algorithms that calculate the quark propagators for the overlap-Dirac fermion operator. The QCD simulations were performed at beta = 5.9 with a lattice volume of 16**3*32., Comment: LATTICE99(Chiral Fermions) 3 pages, 2 figures. A problem with the incorrect definition of the overlap mass has been corrected. The results have changed, but not the conclusions of the paper

We present details of our investigation of the Parallel Tempering algorithm. We consider the application of action matching technology to the selection of parameters. We then present a simple model of the autocorrelations for a particular parallel tempered system. Finally we present numerical results from applying the algorithm to lattice QCD with O(a)-improved dynamical fermions with twin ensemble systems, Comment: 14 pages 2 figures

We argue that lattice simulations of full QCD with varying quark mass are best conducted at fixed lattice spacing rather than at fixed $\beta$. We present techniques which enable this to be carried out effectively, namely the tuning in bare parameter space and efficient stochastic estimation of the fermion determinant. Results and tests of the method are presented. We discuss other applications of such techniques., Comment: LATTICE98(algorithms), 3 pages, 2 eps figures, needs espcrc2.sty

We propose a strategy for conducting lattice QCD simulations at fixed volume but variable quark mass so as to investigate the physical effects of dynamical fermions. We present details of techniques which enable this to be carried out effectively, namely the tuning in bare parameter space and efficient stochastic estimation of the fermion determinant. Preliminary results and tests of the method are presented. We discuss further possible applications of these techniques., Comment: 17 pages, 4 eps figures; affiliation correction in this header + minor post-referee additions

Recent developments and applications of approximate actions for full lattice QCD are described. We present first results based on the stochastic estimation of the fermion determinant on $12^3\times 24$ configurations at $\beta=5.2$., Comment: 3 pages, Latex, no figures, Contribution to Lattice 97, The XV International Symposium on Lattice Field Theory, Edinburgh 22-26 July 1997

We describe a systematic approach to generating approximate actions for the lattice simulation of QCD. Three different tuning conditions are defined to match approximate with true actions, and it is shown that these three conditions become equivalent when the approximate and true actions are sufficiently close. We present a detailed study of approximate actions in the lattice Schwinger model together with an exploratory study of full QCD at unphysical parameter values. We find that the technicalities of the approximate action approach work quite well. However, very delicate tuning is necessary to find an approximate action which gives good predictions for all physical observables. Our best view of the immediate applicability of the methods we describe is to allow high statistics studies of particular physical observables after a low statistics full fermion simulation has been used to prepare the stage., Comment: 34 pages, 5 postscript figures, uses revtex and psfig