Publications (93)160.41 Total impact

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ABSTRACT: We determine the strange quark condensate from lattice QCD for the first time and compare its value to that of the light quark and chiral condensates. The results come from a direct calculation of the expectation value of the trace of the quark propagator followed by subtraction of the appropriate perturbative contribution, derived here, to convert the nonnormalordered $m\bar{\psi}\psi$ to the $\bar{MS}$ scheme at a fixed scale. This is then a welldefined physical `nonperturbative' condensate that can be used in the Operator Product Expansion of currentcurrent correlators. The perturbative subtraction is calculated through $\mathcal{O}(\alpha_s)$ and estimates of higher order terms are included through fitting results at multiple lattice spacing values. The gluon field configurations used are `second generation' ensembles from the MILC collaboration that include 2+1+1 flavors of sea quarks implemented with the Highly Improved Staggered Quark action and including $u/d$ sea quarks down to physical masses. Our results are : $<\bar{s}{s}>^{\bar{MS}}(2 \mathrm{GeV})= (290(15) \mathrm{MeV})^3$, $<\bar{l}{l}>^{\bar{MS}}(2\, \mathrm{GeV})= (283(2) \mathrm{MeV})^3$, where $l$ is a light quark with mass equal to the average of the $u$ and $d$ quarks. The strange to light quark condensate ratio is 1.08(16). The light quark condensate is significantly larger than the chiral condensate in line with expectations from chiral analyses. We discuss the implications of these results for other calculations.Physical review D: Particles and fields 11/2012; 87(3). DOI:10.1103/PhysRevD.87.034503 
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ABSTRACT: 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 (m) over bar (s)(2GeV) = 92.4(1.5) MeV. The MILC result for m(s)/m(l) = 27.2(3) yields (m) over bar (l)(2 GeV) = 3.40(7) MeV for the average of u and d quark masses.Physical Review Letters 04/2010; 104(13):132003. DOI:10.1103/PhysRevLett.104.132003 · 7.73 Impact Factor 
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ABSTRACT: We use lattice QCD simulations, with MILC configurations (including vacuum polarization from u, d, and s quarks), to update our previous determinations of the QCD coupling constant. Our new analysis uses results from 6 different lattice spacings and 12 different combinations of seaquark masses to significantly reduce our previous errors. We also correct for finitelatticespacing errors in the scale setting, and for nonperturbative chiral corrections to the 22 shortdistance quantities from which we extract the coupling. Our final result is alphaV(7.5GeV,nf=3)=0.2120(28), which is equivalent to alphaM Smacr (MZ,nf=5)=0.1183(8). We compare this with our previous result from Wilson loops, which differs by one standard deviation.Physical review D: Particles and fields 12/2008; 78(11). DOI:10.1103/PhysRevD.78.114507 
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ABSTRACT: 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 currentcurrent correlators combined with highorder 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 goldplated 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 
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ABSTRACT: 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 MonteCarlo 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 1419, 2008, Williamsburg, Virginia, USA 
Conference Paper: Quantum Chromodynamics on a SpaceTime Lattice
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ABSTRACT: Quantum chromodynamics has been accepted as the theory of the strong interactions for more than thirty years, ever since the discovery of asymptotic freedom by Gross, Politzer and Wilczek, whose work was recognized with the 2004 Nobel Prize. Despite many successful quantitative predictions for highenergy processes, applications of QCD to stronglycoupled, lowenergy hadronic physics, including such basic quantities as the proton mass, have historically been much less successful. A spacetime lattice discretization of QCD (proposed by Ken Wilson the year after asymptotic freedom) lends itself to direct numerical simulation, but the enormous computational burden of lattice QCD has, until recently, precluded accurate simulations of the full theory. Happily, dramatic improvements in the predictive power of lattice QCD have occurred in the past few years, due to major theoretical progress in our understanding of lattice quantum field theories. These developments are having a significant impact, including the use of lattice QCD to constrain the search for physics beyond the socalled standard model. This talk will give a conceptual review of the theory of QCD at high and low energies and the new developments in lattice QCD.High Performance Computing Systems and Applications, 2008. HPCS 2008. 22nd International Symposium on; 07/2008 
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ABSTRACT: We present oneloop matching coefficients between continuum and lattice QCD for the heavylight fourfermion operators relevant for neutral B meson mixing both within and beyond the standard model. For the lattice theory we use nonrelativistic QCD (NRQCD) to describe b quarks and improved staggered fermions (AsqTad) for light quarks. The gauge action is the treelevel Symanzik improved gauge action. Matching to full QCD is carried out through order alpha(s), Lambda(QCD)/Mb, and alpha(s)/(aM(b)).Physical review D: Particles and fields 06/2008; 77(11). DOI:10.1103/PhysRevD.77.114505 
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ABSTRACT: We use lattice QCD simulations, with MILC gluon configurations and HISQ cquark propagators, to make very precise determinations of moments of charmquark pseudoscalar, vector and axialvector correlators. These moments are combined with new fourloop 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.Physical review D: Particles and fields 06/2008; 78(5). DOI:10.1103/PhysRevD.78.054513 
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ABSTRACT: The effects of unquenching on the perturbative improvement coefficients in the Symanzik action are computed within the framework of LuescherWeisz onshell improvement. We find that the effects of quark loops are surprisingly large, and their omission may well explain the scaling violations observed in some unquenched studies.Physical review D: Particles and fields 08/2007; 76(3):034507034507. DOI:10.1103/PHYSREVD.76.034507 
Article: Progress on Perturbative Matching Calculations for the Charm Quark Mass using the HISQ Action
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ABSTRACT: The highlyimproved staggered quark (HISQ) action is the most accurate discretization scheme to date for the charm quark. Here we report on the progress of perturbative matching for the quark mass using the HISQ action. The matching is done through O(α 2 s) using a combination of Monte Carlo simulations at weak coupling and diagrammatic perturbation theory. When combined with ongoing simulation efforts using the HISQ action, a determination of the charm quark mass to a few percent accuracy can be achieved. Of particular interest will be a comparison with the recent sum rule determination of the charm mass due to Kühn et al. [1]. 
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ABSTRACT: We determine hadronic matrix elements relevant for the mass and width differences, ΔMs and ΔΓs, in the Bs0B̅ s0 meson system using fully unquenched lattice QCD. We employ the MILC Collaboration gauge configurations that include u, d, and s sea quarks using the improved staggered quark (AsqTad) action and a highly improved gluon action. We implement the valence s quark also with the AsqTad action and use nonrelativistic QCD for the valence b quark. For the nonperturbative QCD input into the standard model expression for ΔMs we find fBs√B̂Bs=0.281(21) GeV. Results for fourfermion operator matrix elements entering standard model formulas for ΔΓs are also presented.Physical Review D 01/2007; 76(1). DOI:10.1103/PhysRevD.76.011501 · 4.86 Impact Factor 
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ABSTRACT: 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 heavyheavy and heavylight 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. 
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ABSTRACT: We use perturbative Symanzik improvement to create a new staggeredquark action (HISQ) that has greatly reduced oneloop tasteexchange errors, no treelevel order a^2 errors, and no treelevel order (am)^4 errors to leading order in the quark's velocity v/c. We demonstrate with simulations that the resulting action has tasteexchange interactions that are at least 34 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 psieta_c mass splitting using the HISQ action on lattices where a m_c=0.43 and 0.66, with fullQCD 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 highprecision results for D_s mesons.Physical review D: Particles and fields 11/2006; 75(5). DOI:10.1103/PHYSREVD.75.054502 
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ABSTRACT: Perturbative expansions of several small Wilson loops are computed through nexttonexttoleading order in unquenched lattice QCD, from Monte Carlo simulations at weak couplings. This approach provides a much simpler alternative to conventional diagrammatic perturbation theory, and is applied here for the first time to full QCD. Two different sets of lattice actions are considered: one set uses the unimproved plaquette gluon action together with the unimprovedstaggeredquark action; the other set uses the oneloopimproved Symanzik gaugefield action together with the socalled asqtad improvedstaggeredquark action. Simulations are also done with different numbers of dynamical fermions. An extensive study of the systematic uncertainties is presented, which demonstrates that the small thirdorder perturbative component of the observables can be reliably extracted from simulation data. We also investigate the use of the rational hybrid Monte Carlo algorithm for unquenched simulations with unimprovedstaggered fermions. Our results are in excellent agreement with diagrammatic perturbation theory, and provide an important crosscheck of the perturbation theory input to a recent determination of the strong coupling alphaMS¯(MZ) by the HPQCD collaboration.Physical review D: Particles and fields 05/2006; 73. DOI:10.1103/PhysRevD.73.094512 
Article: Predictive lattice QCD
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ABSTRACT: In the past year, we calculated with lattice QCD three quantities that were unknown or poorly known. They are the q2 dependence of the form factor in semileptonic D → Klν decay, the decay constant of the D meson, and the mass of the Bc meson. In this talk, we summarize these calculations, with emphasis on their (subsequent) confirmation by experiments.International Journal of Modern Physics A 02/2006; 21(04):713719. DOI:10.1142/S0217751X06031934 · 1.09 Impact Factor 
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ABSTRACT: Threeflavor lattice QCD simulations and twoloop perturbation theory are used to make the most precise determination to date of the strange, up, and downquark masses, $m_s$, $m_u$, and $m_d$, respectively. Perturbative matching is required in order to connect the latticeregularized bare quark masses to the masses as defined in the \msbar scheme, and this is done here for the first time at nexttonextto leading (or twoloop) order. The barequark masses required as input come from simulations by the MILC collaboration of a highlyefficient formalism (using socalled ``staggered'' quarks), with three flavors of light quarks in the Dirac sea; these simulations were previously analyzed in a joint study by the HPQCD and MILC collaborations, using degenerate $u$ and $d$ quarks, with masses as low as $m_s/8$, and two values of the lattice spacing, with chiral extrapolation/interpolation to the physical masses. With the new perturbation theory presented here, the resulting \msbar\ masses are $m^\msbar_s(2 {GeV}) = 87(0)(4)(4)(0)$ MeV, and $\hat m^\msbar(2 {GeV}) = 3.2(0)(2)(2)(0)$ MeV, where $\hat m = \sfrac12 (m_u + m_d)$ is the average of the $u$ and $d$ masses. The respective uncertainties are from statistics, simulation systematics, perturbation theory, and electromagnetic/isospin effects. The perturbative errors are about a factor of two smaller than in an earlier study using only oneloop perturbation theory. Using a recent determination of the ratio $m_u/m_d = 0.43(0)(1)(0)(8)$ due to the MILC collaboration, these results also imply $m^\msbar_u(2 {GeV}) = 1.9(0)(1)(1)(2)$ MeV and $m^\msbar_d(2 {GeV}) = 4.4(0)(2)(2)(2)$ MeV. A technique for estimating the next order in the perturbative expansion is also presented, which uses input from simulations at more than one lattice spacing.Physical review D: Particles and fields 12/2005; 73(11). DOI:10.1103/PhysRevD.73.114501 
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ABSTRACT: The HPQCD collaboration has a program for determining the fundamental constants of the Standard Model Lagrangian from lattice QCD. The most accurate method of doing this uses the n_f=2+1 improved staggered MILC ensembles with chiral fitting and multiloop perturbative renormalisation to connect to the continuum \msbar scheme. This program has already been very successful with the recent strong coupling constant determination at threeloops from 28 observables at three lattice spacings, and the oneloop light quark mass calculation last year. Here a preliminary result is presented for the firstever lattice determination of the twoloop multiplicative quark mass renormalisation. The perturbative calculation involved was automated in the generation of the Feynman rules, and the generation and coding of all of the roughly 30 Feynman diagrams. The full formal framework for lattice quark mass renormalisation is given, including the cancellation of infrared divergences in intermediate diagrams. The result was checked by evaluation in three separate gauges and by two authors independently, showing the incredible flexibility and power of this perturbative methodology. Our preliminary result for the twoloop perturbative matching factor, and of systematic errors associated with higherorders, gives \msbar masses at a 2 GeV scale of $m_s = 87(0)(4)(4)(0)$ MeV, and $\frac12(m_u+m_d) = 3.3(0)(2)(2)(0)$ MeV, where the respective uncertainties are from lattice statistical, lattice systematic, perturbative, and electromagnetic and isospin effects. The perturbative errors are a factor of two smaller than in our previous study, and we anticipate reducing this somewhat further from additional analysis of the systematics. 
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ABSTRACT: Using automated perturbation theory techniques, we have computed the oneloop mass of Fermilab fermions, with an improved gluon action. We will present the results of these calculations, and the resulting predictions for the charm and bottom quark masses in the MSbar scheme. We report mc(mc) = 1:22(9) GeV and mb(mb) = 4:7(4) GeV. In addition we present results for the oneloop coeffcients of the Fermilab action. 
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ABSTRACT: We present the first lattice QCD calculation with realistic sea quark content of the D+meson decay constant f(D+). We use the MILC Collaboration's publicly available ensembles of lattice gauge fields, which have a quark sea with two flavors (up and down) much lighter than a third (strange). We obtain f(D+)=201+/3+/17 MeV, where the errors are statistical and a combination of systematic errors. We also obtain f(Ds)=249+/3+/16 MeV for the Ds meson.Physical Review Letters 10/2005; 95(12):122002. DOI:10.1103/PhysRevLett.95.122002 · 7.73 Impact Factor 
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ABSTRACT: We obtain a new value for the QCD coupling constant by combining lattice QCD simulations with experimental data for hadron masses. Our lattice analysis is the first to (1) include vacuum polarization effects from all three lightquark flavors (using MILC configurations), (2) include thirdorder terms in perturbation theory, (3) systematically estimate fourth and higherorder terms, (4) use an unambiguous lattice spacing, and (5) use an [symbol: see text](a2)accurate QCD action. We use 28 different (but related) shortdistance quantities to obtain alpha((5)/(MS))(M(Z)) = 0.1170(12).Physical Review Letters 08/2005; 95(5):052002. DOI:10.1103/PhysRevLett.95.052002 · 7.73 Impact Factor
Publication Stats
2k  Citations  
160.41  Total Impact Points  
Top Journals
Institutions

1993–2012

Simon Fraser University
 Department of Physics
Burnaby, British Columbia, Canada


1993–2006

TRIUMF
Vancouver, British Columbia, Canada


2003

Cornell University
 Laboratory for Elementary Particle Physics
Итак, New York, United States


1986–1991

McGill University
 Department of Physics
Montréal, Quebec, Canada


1989–1990

Massachusetts Institute of Technology
 Department of Physics
Cambridge, Massachusetts, United States
