Search

We investigate numerically the spectral flow introduced by Adams for the staggered Dirac operator on realistic (quenched) gauge configurations. We obtain clear numerical evidence that the definition works as expected: there is a clear separation between crossings near and far away from the origin, and the topological charge defined through the crossings near the origin agrees, for most configurations, with the one defined through the near-zero modes of large taste-singlet chirality of the staggered Dirac operator. The crossings are much closer to the origin if we improve the Dirac operator used in the definition, and they move towards the origin as we decrease the lattice spacing., Comment: References added. Slight changes to the text

In this work we study the two and three-dimensional antiferromagnetic Ising model with an imaginary magnetic field $i\theta$ at $\theta=\pi$. In order to perform numerical simulations of the system we introduce a new geometric algorithm not affected by the sign problem. Our results for the $2D$ model are in agreement with the analytical solutions. We also present new results for the $3D$ model which are qualitatively in agreement with mean-field predictions.

We have developed two methods for handling $b$ quarks in lattice QCD. One uses NRQCD (now improved to include radiative corrections) and the other uses Highly Improved Staggered Quarks (HISQ), extrapolating to the $b$ quark from lighter masses and using multiple lattice spacings to control discretisation errors. Comparison of results for the two different methods gives confidence in estimates of lattice QCD systematic errors, since they are very different in these two cases. Here we show results for heavyonium hyperfine splittings and vector current-current correlator moments using HISQ quarks, to add to earlier results testing the heavy HISQ method with pseudoscalar mesons. We also show the form factor for $B \rightarrow \pi l \nu$ decay at zero recoil using NRQCD $b$ quarks and $u/d$ quarks with physical masses. This allows us to test the soft pion theorem relation ($f_0(q^2_{max})=f_B/f_{\pi}$) accurately and we find good agreement as $M_{\pi} \rightarrow 0$. }, Comment: 5 pages, 3 figures, Proceedings of Lattice2013

We analyze the vacuum structure of a generalized lattice Nambu--Jona-Lasinio model with two flavors of Wilson fermions, such that its continuum action is the most general four-fermion action with 'trivial' color interactions, and having a $SU(2)_V x SU(2)_A$ symmetry in the chiral limit. The phase structure of this model in the space of the two four-fermion couplings shows, in addition to the standard Aoki phases, new phases with $<\bar\psi \gamma_5\psi> != 0$, in close analogy to similar results recently suggested by some of us for lattice QCD with two degenerate Wilson fermions. This result shows how the phase structure of an effective model for low energy QCD cannot be entirely understood from Wilson Chiral Perturbation Theory, based on the standard QCD chiral effective Lagrangian approach., Comment: 24 pages, 3 figures, sent to Nuclear Physics B

We present a new study of the form factors for D -> K semileptonic decay from lattice QCD that allows us to compare the shape of the vector form factor to experiment and, for the first time, to extract V_cs using results from all experimental q^2 bins. The valence quarks are implemented with the Highly Improved Staggered Quark action on MILC configurations that include u, d and s sea quarks. The scalar and vector currents are nonperturbatively normalised and, using phased boundary conditions, we are able to cover the full q^2 range accessible to experiment. Our result is V_cs = 0.963(5)_{expt}(14)_{lattice}. We also demonstrate that the form factors are insensitive to whether the spectator quark is u/d or s, which has implications for other decay channels., Comment: 5 pages, 6 figures

We show results from calculations in full lattice QCD of the mass, leptonic width and radiative decay rate to eta_c of the J/psi meson. These provide few % tests of QCD. Another (1.5%) test comes from comparison of time-moments of the vector charmonium correlator with results derived from the experimental values of R(e+e- to hadrons) in the charm region., Comment: 3 pages, 4 figures, Proceedings of Confinement2012

We study the two-dimensional Antiferromagnetic Ising Model with an imaginary magnetic field i\theta at \theta=\pi. We use a new geometric algorithm which does not present a sign problem. This allows us to perform efficient numerical simulations of this system., Comment: 7 pages, 3 figures, submitted as a contribution to the 30th International Symposium on Lattice Field Theory, June 24-29, 2012, Cairns, Australia

We calculate the J/{\psi} mass, leptonic width and radiative decay rate to \gamma \eta_c from lattice QCD including u, d and s quarks in the sea for the first time. We use the Highly Improved Staggered Quark formalism and nonperturbatively normalised vector currents for the leptonic and radiative decay rates. Our results are: M_{J/\psi} -M_{\eta_c} = 116.5(3.2) MeV; \Gamma(J/{\psi} to e^+e^-) = 5.48(16) keV; \Gamma(J/{\psi} to \gamma \eta_c) = 2.49(19) keV. The first two are in good agreement with experiment, with \Gamma(J/{\psi} to e^+e^-) providing a test of a decay matrix element in QCD, independent of CKM uncertainties, to 2%. At the same time results for the time moments of the correlation function can be compared to values from the charm contribution to \Gamma(e^+e^- to hadrons), giving a 1.5% test of QCD. Our results show that an improved experimental error would enable a similarly strong test from \Gamma(J/{\psi} to \gamma \eta_c)., Comment: 20 pages, 16 figures. Added some text plus a figure and two tables comparing the time moments of the charmonium vector correlator to results extracted from experiment for the charm contribution to e^+e^- -> hadrons. This allows a 1.5% test of QCD. Further minor amendments to the text. Version accepted for publication by Physical Review D

We determine masses and decay constants of heavy-heavy and heavy-charm pseudoscalar mesons as a function of heavy quark mass using a fully relativistic formalism known as Highly Improved Staggered Quarks for the heavy quark. We are able to cover the region from the charm quark mass to the bottom quark mass using MILC ensembles with lattice spacing values from 0.15 fm down to 0.044 fm. We obtain f_{B_c} = 0.427(6) GeV; m_{B_c} = 6.285(10) GeV and f_{\eta_b} = 0.667(6) GeV. Our value for f_{\eta_b} is within a few percent of f_{\Upsilon} confirming that spin effects are surprisingly small for heavyonium decay constants. Our value for f_{B_c} is significantly lower than potential model values being used to estimate production rates at the LHC. We discuss the changing physical heavy-quark mass dependence of decay constants from heavy-heavy through heavy-charm to heavy-strange mesons. A comparison between the three different systems confirms that the B_c system behaves in some ways more like a heavy-light system than a heavy-heavy one. Finally we summarise current results on decay constants of gold-plated mesons., Comment: 16 pages, 12 figures

We investigate numerically the spectral flow introduced by Adams for the staggered Dirac operator on realistic gauge configurations. We study both the unimproved and the HISQ Dirac operators. We compare the spectral flow index with the index obtained by identifying low-lying modes of large chirality., Comment: Talk presented at The XXIX International Conference on Lattice Field Theory - Lattice 2011, July 10-16, 2011, Squaw Valley, Lake Tahoe, California

We give results for the Upsilon spectrum from lattice QCD using an improved version of the NRQCD action for b quarks which includes radiative corrections to kinetic terms at O(v^4) in the velocity expansion. We also include for the first time the effect of up, down, strange and charm quarks in the sea using 'second generation' gluon field configurations from the MILC collaboration. Using the {\Upsilon} 2S - 1S splitting to determine the lattice spacing, we are able to obtain the 1P - 1S splitting to 1.4% and the 3S - 1S splitting to 2.4%. Our improved result for M (Upsilon) - M (eta_b) is 70(9) MeV and we predict M(Upsilon) - M(eta_b') = 35(3) MeV. We also calculate pi, K and eta_s correlators using the Highly Improved Staggered Quark action and perform a chiral and continuum extrapolation to give values for M(eta_s) (0.6893(12) GeV) and f_{eta_s} (0.1819(5) GeV) that allow us to tune the strange quark mass as well as providing an independent and consistent determination of the lattice spacing. Combining the NRQCD and HISQ analyses gives mb/ms = 54.7(2.5) and a value for the heavy quark potential parameter of r_1 = 0.3209(26) fm., Comment: 38 pages, many figures

We present a new determination of the $B_s$ leptonic decay constant from lattice QCD simulations that use gluon configurations from MILC and a highly improved discretization of the relativistic quark action for both valence quarks. Our result, $f_{B_s} = 0.225(4)$\,GeV, is almost three times more accurate than previous determinations. We analyze the dependence of the decay constant on the heavy quark's mass and obtain the first empirical evidence for the leading $1/\sqrt{m_h}$ dependence predicted by Heavy Quark Effective Theory (HQET). As a check, we use our analysis technique to calculate the $m_{B_s}-m_{\eta_b}/2$ mass difference. Our result agrees with experiment to within errors of $11\,\mathrm{MeV}$ (better than 2%). We discuss how to extend our analysis to other quantities in $B_s$ and $B$ physics, making 2%-precision possible for the first time., Comment: 5 pages, 3 figures

We present a study of $D \rightarrow K, l \nu$ semileptonic decays on the lattice which employs the HISQ action for both the charm and the light quarks. We work with MILC unquenched $N_f = 2 + 1$ lattices and determine the scalar form factor $f_0(q^2)$. This form factor is obtained from a scalar current matrix element that does not require any operator matching. We find $f^{D \rightarrow K}_0(0) \equiv f^{D \rightarrow K}_+(0) = 0.747(19)$ in the chiral plus continuum limit and hereby improve the theory error on this quantity by a factor of $\sim$4 compared to previous lattice determinations. Combining the new theory result with recent experimental measurements of the product $f^{D \rightarrow K}_+(0) * |V_{cs}| $ from BaBar and CLEO-c leads to a very precise direct determination of the CKM matrix element $|V_{cs}| $, $|V_{cs}| = 0.961(11)(24)$, where the first error comes from experiment and the second is the lattice QCD theory error., Comment: 7 pages, 6 figures, and 1 table. Talk given at Lattice 2010, June 14-19, 2010, Villasimius, Italy

In order to elucidate the vacuum structure of the Aoki phase, we carried out a numerical investigation of QCD with two flavours of Wilson fermions, within the p.d.f. framework and in the absence of external sources. The simulations performed at V = 44 suggest a rich vacuum structure, where the observable $i\bar\psi\gamma_5\psi$ is allowed to take non-zero values of the same order of magnitude than the order parameter of the Aoki phase $i\bar\psi\gamma_5\tau_3\psi$. However, the simulation at higher volumes V = 6^4 suffers from large statistical errors., Comment: 7 pages, talk given at the XXVIII International Symposium on Lattice Field Theory, June 14-19, 2010, Villasimius, Italy. v2 corrects agreements

We update our previous determination of both the decay constant and the mass of the $D_s$ meson using the Highly Improved Staggered Quark formalism. We include additional results at two finer values of the lattice spacing along with improved determinations of the lattice spacing and improved tuning of the charm and strange quark masses. We obtain $m_{D_s}$ = 1.9691(32) GeV, in good agreement with experiment, and $f_{D_s}$ = 0.2480(25) GeV. Our result for $f_{D_s}$ is 1.6$\sigma$ lower than the most recent experimental average determined from the $D_s$ leptonic decay rate and using $V_{cs}$ from CKM unitarity. Combining our $f_{D_s}$ with the experimental rate we obtain a direct determination of $V_{cs} = 1.010(22)$, or alternatively $0.990 {+0.013 \atop -0.016}$ using a probability distribution for statistical errors for this quantity which vanishes above 1. We also include an accurate prediction of the decay constant of the $\eta_c$, $f_{\eta_c}$ = 0.3947(24) GeV, as a calibration point for other lattice calculations., Comment: 24 pages, 20 figures. Updated to include new experimental results from BaBar, new experimental averages from HFAG and consequent discussion of theory/experiment comparison. Other minor typographical changes. Version accepted by Phys. Rev. D

We extend our earlier lattice-QCD analysis of heavy-quark correlators to smaller lattice spacings and larger masses to obtain new values for the c mass and QCD coupling, and, for the first time, values for the b mass: m_c(3GeV,n_f=4)=0.986(6)GeV, alpha_msb(M_Z,n_f=5)=0.1183(7), and m_b(10GeV,n_f=5)=3.617(25)GeV. These are among the most accurate determinations by any method. We check our results using a nonperturbative determination of the mass ratio m_b(mu,n_f)/m_c(mu,n_f); the two methods agree to within our 1% errors and taken together imply m_b/m_c=4.51(4). We also update our previous analysis of alpha_msb from Wilson loops to account for revised values for r_1 and r_1/a, finding a new value alpha_\msb(M_Z,n_f=5)=0.1184(6); and we update our recent values for light-quark masses from the ratio m_c/m_s. Finally, in the Appendix, we derive a procedure for simplifying and accelerating complicated least-squares fits., Comment: 16 pages, 10 figures, 3 tables

The Vafa-Witten arguments on the realization of parity and flavour symmetries in the QCD vacuum do not apply to two lattice regularizations of QCD which are able to reproduce the chiral anomaly: Wilson fermions and Ginsparg-Wilson fermions. We show here how using the last regularization one can get, from first principles, that the more standard order parameters for these symmetries take a vanishing vacuum expectation value for any number of flavours, if quarks have a non-vanishing mass., Comment: 14 pages. References added

We review recent results for B_d and B_s mixing parameters using MILC N_f=2+1 lattices, NRQCD b-quarks and AsqTad light quarks. Latest numbers for decay constants f_B and f_{B_s} are also presented. Combining our lattice results with experimental determinations of the mass differences \Delta M_d and \Delta M_s leads to an important ratio of elements of the CKM matrix, |V_{td}|/|V_{ts}| = 0.214(1)(5) and an updated Standard Model number for the branching fraction Br(B_s -> mu^+ mu^-) = 3.19(19) x 10^{-9}. Preliminary new results for f_{B_s} based on other actions are also described., Comment: 7 pages, 5 figures, contribution to the 27th International Symposium on Lattice Field Theory, Beijing, July 2009

We present a program to study D semi-leptonic decay form factors with HISQ charm and light quarks. In this exploratory work, we study $D_s$ to $\eta_s$, $l\nu_l$ semi-leptonic decay on MILC coarse ($20^3\times64$) gauge configurations with 2+1 dynamical flavors. We develop a new method to calculate $f_+(0)$ and $f_0(q^2)$ from scalar current matrix elements, $<\pi|\bar{s}c|D>$, which does not require any operator renormalization. We also determine $f_+(q^2)$ and $f_0(q^2)$ from vector currents with a fully non-perturbative renormalization method., Comment: 7 pages, 3 figures, Talk given at Lattice 2009, July 26-31 2009, Peking University, Beijing, China

By using a single formalism to handle charm, strange and light valence quarks in full lattice QCD for the first time, we are able to determine ratios of quark masses to 1%. For $m_c/m_s$ we obtain 11.85(16), an order of magnitude more precise than the current PDG average. Combined with 1% determinations of the charm quark mass now possible this gives $\bar{m}_s(2{\rm GeV}) =$ 92.4(1.5) MeV. The MILC result for $m_s/m_l = 27.2(3) $ yields $\bar{m}_l(2{\rm GeV})$ = 3.40(7) MeV for the average of $u$ and $d$ quark masses., Comment: 4 pages, 2 figures. Version accepted by Physical Review Letters. Changes include modifying the title, using the MILC value for m_s/m_l which changes slightly the resulting up and down quark masses and their average, adding some references and making other small adjustments to the text for space reasons.

We discuss the status of our ongoing efforts to improve on our calculation of the $D_s$ decay constant. We show preliminary results on the ratio of the charm to the strange quark mass. We also present preliminary results for spectroscopy, decay constants and bottom quark mass obtained by performing calculations with highly improved staggered quarks at masses above the c mass and close to the b mass., Comment: 8 pages. Contribution to 27th International Symposium on Lattice Field Theory, Beijing, China, 2009

We compare three different methods to determine the lattice spacing in lattice QCD and give results from calculations on the MILC ensembles of configurations that include the effect of $u$, $d$ and $s$ sea quarks. It is useful, for ensemble to ensemble comparison, to express the results as giving a physical value for $r_1$, a parameter from the heavy quark potential. Combining the three methods gives a value for $r_1$ in the continuum limit of 0.3133(23)(3) fm. Using the MILC values for $r_0/r_1$, this corresponds to a value for the $r_0$ parameter of 0.4661(38) fm. We also discuss how to use the $\eta_s$ for determining the lattice spacing and tuning the $s$-quark mass accurately, by giving values for $m_{\eta_s}$ (0.6858(40) GeV) and $f_{\eta_s}$ (0.1815(10) GeV)., Comment: 15 pages

The vacuum structure of QCD with two degenerated flavours of Wilson fermions is discussed by means of the p.d.f. (probability distribution function) formalism. Under certain assumptions, either new phases related to Aoki's appear, characterized by a non-vanishing expectation value of the condensate $i\bar\psi_u\gamma_5\psi_u + i\bar\psi_d\gamma_5\psi_d$, enriching the standard picture of the QCD vacuum structure with Wilson fermions; or the eigenvalues of the Dirac-Wilson operator must comply with a non-trivial, infinite set of sum rules, enforcing the currently accepted structure of the Aoki vacuum. The same scenario is analyzed using the Ginsparg-Wilson regularization. In this case, the absence of any parity and/or flavour breaking phase is proved for a non-zero value of the fermion mass., Comment: 7 pages, no figures, to be published in PoS(Lattice'09)

By using the Highly Improved Staggered Quark formalism to handle charm, strange and light valence quarks in full lattice QCD, and NRQCD to handle bottom valence quarks we are able to determine accurately ratios of the B meson vector-pseudoscalar mass splittings, in particular, (m(B*_c)-m(B_c))/(m(B*_s)-m(B_s)). We find this ratio to be 1.15(15), showing the `light' quark mass dependence of this splitting to be very small. Hence we predict m(B_c*) = 6.330(7)(2)(6) GeV where the first two errors are from the lattice calculation and the third from existing experiment. This is the most accurate prediction of a gold-plated hadron mass from lattice QCD to date., Comment: 4 pages, 2 figures

We present an update of results from the HPQCD collaboration on charm physics using the Highly Improved Staggered Quark action. This includes a precise determination of m_c using moments of current-current correlators combined with high-order continuum QCD perturbation theory. We also include an update on the determination of alpha_s from lattice QCD, preliminary results on the determination of m_b and a summary plot of the status of the gold-plated meson spectrum. There is an appendix on tackling systematic errors in fitting using the Bayesian approach., Comment: 14 pages, 7 figures, combined proceedings of talks by Christine Davies and Peter Lepage at the International Symposium on Lattice Field Theory 2008, Williamsburg

We give improved results for B meson masses using NRQCD $b$ quarks and HISQ light valence quarks for a range of lattice spacings and sea quark masses enabling controlled extrapolation to the physical point., Comment: 6 pages, 3 figures, poster presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We provide a new determination of the charm quark mass using the Highly Improved Staggered Quark (HISQ) action, finding m_c(3 GeV) = 0.983(23) GeV. Our determination makes extensive use of second order lattice perturbation theory in matching the bare lattice mass to the MSbar scheme. This matching utilises both traditional diagrammatic perturbation theory and weak coupling simulations. The second of these techniques allows us to extract perturbative coefficients from Monte-Carlo simulations and the process of doing this is laid out in some detail here., Comment: 7 pages, 4 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We use lattice QCD simulations, with MILC gluon configurations and HISQ c-quark propagators, to make very precise determinations of moments of charm-quark pseudoscalar, vector and axial-vector correlators. These moments are combined with new four-loop results from continuum perturbation theory to obtain several new determinations of the MSbar mass of the charm quark and of the MSbar coupling. We find m_c(3GeV)=0.986(10)GeV, or, equivalently, m_c(m_c)=1.268(9)GeV, both for n_f=4 flavors; and alpha_msb(3GeV,n_f=4)=0.251(6), or, equivalently, alpha_\msb(M_Z,n_f=5)=0.1174(12). The new mass agrees well with results from continuum analyses of the vector correlator using experimental data for e+e- annihilation (instead of using lattice QCD simulations). These lattice and continuum results are the most accurate determinations to date of this mass. Ours is also one of the most accurate determinations of the QCD coupling by any method., Comment: 12 pages, 5 figures, 5 tables

We explore the possibility of studying $B\to\pi l\nu$ semileptonic decays at large recoil momentum. Our methods include the use of a random-wall source for the pion to reduce statistical errors, and different smearing functions are used for the B meson to improve the overlap with the ground state. We observe, in general, a factor of 3-4 improvement in the signal-to-noise ratio in correlation functions if random-wall propagators are used., Comment: 7 pages, poster presented at the XXV International Symposium on Lattice Field Theory, 30 July - 4 August 2007, Regensburg, Germany

We determine $D$ and $D_s$ decay constants from lattice QCD with 2% errors, 4 times better than experiment and previous theory: $f_{D_s}$ = 241(3) MeV, $f_D$ = 207(4) MeV and $f_{D_s}/f_D$ = 1.164(11). We also obtain $f_K/f_{\pi}$ = 1.189(7) and $(f_{D_s}/f_D)/(f_K/f_{\pi})$ = 0.979(11). Combining with experiment gives $V_{us}$=0.2262(14) and $V_{cs}/V_{cd}$ of 4.43(41). We use a highly improved quark discretization on MILC gluon fields that include realistic sea quarks fixing the $u/d, s$ and $c$ masses from the $\pi$, $K$, and $\eta_c$ meson masses. This allows a stringent test against experiment for $D$ and $D_s$ masses for the first time (to within 7 MeV)., Comment: 4 pages, 2 figures. Published version - changes from original include a more extensive discussion of errors and an error budget table covering more quantities. There are very small changes in some of the values reported

We use a relativistic highly improved staggered quark action to discretize charm quarks on the lattice. We calculate the masses and the dispersion relation for heavy-heavy and heavy-light meson states, and show that for lattice spacings below .1 fm, the discretization errors are at the few percent level. We also discuss the prospects for accurate calculations at the few percent level of f_D_s, f_D, and the leptonic width of the psi and phi., Comment: 7 pages, contribution to Lattice 2006

We use perturbative Symanzik improvement to create a new staggered-quark action (HISQ) that has greatly reduced one-loop taste-exchange errors, no tree-level order a^2 errors, and no tree-level order (am)^4 errors to leading order in the quark's velocity v/c. We demonstrate with simulations that the resulting action has taste-exchange interactions that are at least 3--4 times smaller than the widely used ASQTAD action. We show how to estimate errors due to taste exchange by comparing ASQTAD and HISQ simulations, and demonstrate with simulations that such errors are no more than 1% when HISQ is used for light quarks at lattice spacings of 1/10 fm or less. The suppression of (am)^4 errors also makes HISQ the most accurate discretization currently available for simulating c quarks. We demonstrate this in a new analysis of the psi-eta_c mass splitting using the HISQ action on lattices where a m_c=0.43 and 0.66, with full-QCD gluon configurations (from MILC). We obtain a result of~111(5) MeV which compares well with experiment. We discuss applications of this formalism to D physics and present our first high-precision results for D_s mesons., Comment: 21 pages, 8 figures, 5 tables

We study various improved staggered quark Dirac operators on quenched gluon backgrounds in lattice QCD. We find a clear separation of the spectrum of eigenvalues into high chirality, would-be zero modes and others, in accordance with the Index Theorem. We find the expected clustering of the non-zero modes into quartets as we approach the continuum limit. The predictions of random matrix theory for the epsilon regime are well reproduced. We conclude that improved staggered quarks near the continuum limit respond correctly to QCD topology., Comment: 6 pages, 2 figures, talk presented at Lattice 2005 (Dublin). To appear in Proceedings of Science

We investigate and clarify the role of topology and the issues surrounding the epsilon regime for staggered quarks. We study unimproved and improved staggered quark Dirac operators on quenched lattice QCD gluon backgrounds generated using a Symanzik-improved gluon action. For the improved Dirac operators we find a clear separation of the spectrum into would-be zero modes and others. The number of would-be zero modes depends on the topological charge as predicted by the continuum Index Theorem, and the expectation values of their chirality are large for the most improved actions (approx 0.7). The remaining modes have low chirality and show clear signs of clustering into quartets that become degenerate in the continuum limit. We demonstrate that the lattice spacing and volume dependence of the eigenvalues follow expectations. Furthermore, the non-zero modes follow the random matrix theory predictions for all topological charge sectors. The values of the chiral condensate extracted from fits to the theoretical distributions are consistent with each other, and with the results obtained from the total density of eigenvalues using the Banks-Casher relation. We conclude that staggered quarks respond correctly to QCD topology when both fermion and gauge actions are improved., Comment: 17 pages, a few typos corrected, part of one figure changed

We study various improved staggered quark Dirac operators on quenched gluon backgrounds in lattice QCD generated using a Symanzik-improved gluon action. We find a clear separation of the spectrum of eigenvalues into would-be zero modes and others. The number of would-be zero modes depends on the topological charge as expected from the Index Theorem, and their chirality expectation value is large. The remaining modes have low chirality and show clear signs of clustering into quartets and approaching the random matrix theory predictions for all topological charge sectors. We conclude that improvement of the fermionic and gauge actions moves the staggered quarks closer to the continuum limit where they respond correctly to QCD topology., Comment: Lattice2004(plenary)

We compare several improved actions for staggered quarks. We study the effect of improvement on the taste changing interactions by calculating the splitting in the pion spectrum. We investigate the effect of the improvement on some topological properties., Comment: 3 pages, 3 figures, Lattice 2003 proceedings

We study various improved staggered quark Dirac operators on quenched gluon backgrounds in lattice QCD generated using a Symanzik-improved gluon action. We find a clear separation of the spectrum into would-be zero modes and others. The number of would-be zero modes depends on the topological charge as expected from the Index Theorem, and their chirality expectation value is large (approximately 0.7). The remaining modes have low chirality and show clear signs of clustering into quartets and approaching the random matrix theory predictions for all topological charge sectors. We conclude that improvement of the fermionic and gauge actions moves the staggered quarks closer to the continuum limit where they respond correctly to QCD topology., Comment: 4 pages, 3 figures

We argue that high-precision lattice QCD is now possible, for the first time, because of a new improved staggered quark discretization. We compare a wide variety of nonperturbative calculations in QCD with experiment, and find agreement to within statistical and systematic errors of 3% or less. We also present a new determination of alpha_msbar(Mz); we obtain 0.121(3). We discuss the implications of this breakthrough for phenomenology and, in particular, for heavy-quark physics., Comment: 2 figures, revtex

We describe the first lattice determination of the strong coupling constant with 3 flavors of dynamical quarks. The method follows previous analyses in using a perturbative expansion for the plaquette and Upsilon spectroscopy to set the scale. Using dynamical configurations from the MILC collaboration with 2+1 flavors of dynamical quarks we are able to avoid previous problems of having to extrapolate to 3 light flavors from 0 and 2. Our results agree with our previous work: alpha_s_MSbar(M_Z) = 0.121(3)., Comment: Lattice2002(heavyquark)

We describe the bottomonium spectrum obtained on the MILC configurations which incorporate 2+1 flavors of dynamical quarks. We compare to quenched and 2 flavor results also on MILC configurations. We show that the lattice spacing determination using different quantities shows clear signs of convergence with 2+1 flavors and give results for the leptonic width and hyperfine splitting, in the form of the ratio of the 1st excited state of the Upsilon to that of the ground state., Comment: 3 pages, 3 figures, Lattice2002(heavyquark)

We examine the gluon propagator in the Laplacian gauge in quenched lattice QCD as a function of the number of colours. We observe a weak dependence on $N_c$ over the whole momentum range. This implies an almost $N_c$-independent gluon pole mass in units of the string tension., Comment: 5 pages, 7 figures. A study of the gluon propagator in SU(4) for various lattice sizes was added. Extended version to appear in Phys. Rev. D

We extend our study of the gluon propagator in quenched lattice QCD using the Laplacian gauge to a finer lattice. We verify the existence of a pole mass as we take the continuum limit and deduce a value of $\sim 600^{+150}_{-30}$ MeV for this pole mass. We find a finite value of $(454(5){\rm MeV})^{-2}$ for the renormalized zero-momentum propagator, in agreement with results on coarser lattices., Comment: 5 pages, 5 figures. Extended version to appear in Phys. Rev. D

We compute the ratio between the scale $\Lambda_L$ associated with a lattice formulation of QCD using the overlap-Dirac operator, and $\Lambda_{MS-bar}$. To this end, the one-loop relation between the lattice coupling $g_0$ and the coupling renormalized in the MS-bar scheme is calculated, using the lattice background field technique. We also compute the one-loop renormalization $Z_\Gamma$ of the two-quark operators $\bar{\psi} \Gamma \psi$, where $\Gamma$ denotes a generic Dirac matrix. Furthermore, we study the renormalization of quark bilinears which are more extended and have better chiral properties. Finally, we present improved estimates of $Z_\Gamma$, coming from cactus resummation and from mean field perturbation theory., Comment: 3 pages, LATeX with 2 PS figure files. Talk presented at Confinement IV, Vienna, July 2000. Longer write-ups in: hep-lat/9909158 (NPB 571 (2000) 257), hep-lat/0002010 (NPB 580 (2000) 394). One further reference added, no other changes

We present lattice results for the gluon propagator for SU(2) and SU(3) in the Laplacian gauge which avoids lattice Gribov copies. In SU(3) we compare with the most recent lattice calculation in Landau gauge and with various approximate solutions of the Dyson Schwinger equations (DSE)., Comment: 3 pages, 3 figures; Talk presented at Confinement IV, July 3-8, 2000, Vienna, Austria. An additional line of explanation regarding the gauge fixing procedure was added

We study the gluon propagator in quenched lattice QCD using the Laplacian gauge which is free of lattice Gribov copies. We compare our results with those obtained in the Landau gauge on the lattice, as well as with various approximate solutions of the Dyson Schwinger equations. We find a finite value $\sim (445 \rm{MeV})^{-2}$ for the renormalized zero-momentum propagator (taking our renormalization point at 1.943 GeV), and a pole mass $\sim 640 \pm 140$ MeV., Comment: Discussion of the renormalized gluon propagator and of the Laplacian gauge fixing procedure extended. Version to appear in Phys. Rev. D. 15 pages, 8 figures

We calculate the critical value of the hopping parameter, $\kappa_c$, in Lattice QCD with Wilson fermions, to two loops in perturbation theory. This quantity is an additive renormalization; as such, it is characterized not only by the standard caveats regarding the asymptotic nature of perturbative results, but also by a linear divergence in the lattice spacing. Consequently, our calculation tests rather stringently the limits of applicability of perturbation theory. We compare our results to non perturbative evaluations of $\kappa_c$ coming from Monte Carlo simulations. Finally, we apply a tadpole improvement technique on our results; this shifts them quite favourably towards the non perturbative values., Comment: 9 pages, LATeX with 2 EPS figures

We compute the one-loop lattice renormalization of the two-quark operators $\bar{\psi} \Gamma \psi$, where $\Gamma$ denotes the generic Dirac matrix, for the lattice formulation of QCD using the overlap-Dirac operator. We also study the renormalization of quark bilinears which are more extended and have better chiral properties. Finally, we present improved estimates of these renormalization constants, coming from cactus resummation and from mean field perturbation theory., Comment: 14 pages, LATeX with 2 PS figure files. One further reference on the axial anomaly has been added; no other changes. Nuclear Physics B, in press

We introduce a lattice model with local U(1) gauge symmetry which incorporates explicit frustration in d >2. The form of the action is inspired from the loop expansion of the fermionic determinant in standard lattice QED. We study through numerical simulations the phase diagram of the model, revealing the existence of a frustrated (antiferromagnetic) phase for d=3 and d=4, once an appropriate order parameter is identified., Comment: 7 pages, 7 figures

We study the relaxational dynamics of the one-spin facilitated Ising model introduced by Fredrickson and Andersen. We show the existence of a critical time which separates an initial regime in which the relaxation is exponentially fast and aging is absent from a regime in which relaxation becomes slow and aging effects are present. The presence of this fast exponential process and its associated critical time is in agreement with some recent experimental results on fragile glasses., Comment: 20 Pages + 7 Figures, Revtex