Q. Mason

Highly improved staggered quarks on the lattice, with applications to charm physics

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{sup 2} errors, and no tree-level order (am){sup 4} errors to leading order in the quarks velocity v/c. We demonstrate with simulations that the resulting action has taste-exchange interactions that are 3-4 times smaller than the widely used ASQTAD action. We show how to bound errors due to taste exchange by comparing ASQTAD and HISQ simulations, and demonstrate with simulations that such errors are likely no more than 1% when HISQ is used for light quarks at lattice spacings of 1/10 fm or less. The suppression of (am){sup 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}{sub c} mass splitting using the HISQ action on lattices where am{sub 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 the experiment. We discuss applications of this formalism to D physics and present our first high-precision results for D{sub s} mesons.

B-meson decay constant from unquenched lattice QCD.

We present determinations of the -meson decay constant f(B) and f(B)(s)/f(B) using the MILC Collaboration unquenched gauge configurations, which include three flavors of light sea quarks. The mass of one of the sea quarks is kept around the strange quark mass, and we explore a range in masses for the two lighter sea quarks down to m(s)/8. The heavy quark is simulated using nonrelativistic QCD, and both the valence and sea light quarks are represented by the highly improved (AsqTad) staggered quark action. The good chiral properties of the latter action allow for a more accurate chiral extrapolation to physical up and down quarks than has been possible in the past. We find f(B)=216(9)(19)(4)(6) MeV and f(B)(s)/f(B)=1.20(3)(1).

First determination of the strange and light quark masses from full lattice QCD

We compute the strange quark mass

Accurate determinations of alpha(s) from realistic lattice QCD.

{m}_{s}

Low-lying Dirac spectrum of staggered quarks

and the average of the u and d quark masses

Unquenching effects on the coefficients of the Lüscher-Weisz action

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Further improvements to staggered quarks

using full lattice QCD with three dynamical quarks combined with the experimental values for the

Second order perturbation theory for improved gluon and staggered quark actions

\ensuremath{\pi}

Improvement and taste symmetry breaking for staggered quarks

and K masses. The simulations have degenerate u and d quarks with masses

Mass renormalisation for improved staggered quarks

{m}_{u}{=m}_{d}\ensuremath{\equiv}\mathrm{m\ifmmode \hat{}\else \^{}\fi{}}