410 research outputs found

### Mass renormalisation for improved staggered quarks

Improved staggered quark actions are designed to suppress flavour changing
strong interactions. We discuss the perturbation theory for this type of
actions and show the improvements to reduce the quark mass renormalisation
compared to naive staggered quarks. The renormalisations are of similar size as
for Wilson quarks.Comment: LaTeX, 3 pages, Lattice2001(spectrum

### Scale Setting for $\alpha_s$ Beyond Leading Order

We present a general procedure for applying the scale-setting prescription of
Brodsky, Lepage and Mackenzie to higher orders in the strong coupling constant
\alphas. In particular, we show how to apply this prescription when the
leading coefficient or coefficients in a series in \alphas are anomalously
small. We give a general method for computing an optimum scale numerically,
within dimensional regularization, and in cases when the coefficients of a
series are known. We find significant corrections to the scales for $R_{e^+
e^-}$, $\Gamma(B \to X_u e \bar{\nu})$, $\Gamma(t \to b W)$, and the ratios of
the quark pole to \MSbar and lattice bare masses.Comment: Lattice 2000 (Perturbation Theory), 5 pages, 7 figures, LaTe

### Irreducible Multiplets of Three-Quark Operators on the Lattice: Controlling Mixing under Renormalization

High luminosity accelerators have greatly increased the interest in
semi-exclusive and exclusive reactions involving nucleons. The relevant
theoretical information is contained in the nucleon wavefunction and can be
parametrized by moments of the nucleon distribution amplitudes, which in turn
are linked to matrix elements of three-quark operators. These can be calculated
from first principles in lattice QCD. However, on the lattice the problems of
operator mixing under renormalization are rather involved. In a systematic
approach we investigate this issue in depth. Using the spinorial symmetry group
of the hypercubic lattice we derive irreducibly transforming three-quark
operators, which allow us to control the mixing pattern.Comment: 13 page

### QCD on Coarse Lattices

We show that the perturbatively-improved gluon action for QCD, once it is
tadpole-improved, gives accurate results even with lattice spacings as large as
0.4~fm. {\em No\/} tuning of the couplings is required. Using this action and
lattice spacing, we obtain a static potential that is rotationally invariant to
within a few percent, the spin-averaged charmonium spectrum accurate to within
30--40~MeV, and scaling to within 5--10\%. We demonstrate that simulations on
coarse lattices are several orders of magnitude less costly than simulations
using current methods.Comment: 4 page

### Perturbation theory vs. simulation for tadpole improvement factors in pure gauge theories

We calculate the mean link in Landau gauge for Wilson and improved SU(3)
anisotropic gauge actions, using two loop perturbation theory and Monte Carlo
simulation employing an accelerated Langevin algorithm. Twisted boundary
conditions are employed, with a twist in all four lattice directions
considerably improving the (Fourier accelerated) convergence to an improved
lattice Landau gauge. Two loop perturbation theory is seen to predict the mean
link extremely well even into the region of commonly simulated gauge couplings
and so can be used remove the need for numerical tuning of self-consistent
tadpole improvement factors. A three loop perturbative coefficient is inferred
from the simulations and is found to be small. We show that finite size effects
are small and argue likewise for (lattice) Gribov copies and double Dirac
sheets.Comment: 13 pages of revtex

### F_B from moving B mesons

We show results for the B meson decay constant calculated both for B mesons
at rest and those with non-zero momentum and using both the temporal and
spatial components of the axial vector current. It is an important check of
lattice systematic errors that all these determinations of f_B should agree. We
also describe how well different smearings for the B meson work at non-zero
momentum - the optimal smearing has a narrow smearing for the b quark.Comment: Lattice2001(heavyquark

### Semileptonic B Decays from an NRQCD/D234 Action

Semileptonic B decays are studied on quenched anisotropic lattices using
Symanzik improved glue, NRQCD heavy quark and D234 light quark actions. We
employ constrained fits to extract ground state contributions to two- and
three-point correlators. Results for the B --> pi, l nubar decay form factors
are compared with previous lattice results. We find that our systematic errors
(excluding quenching errors) are dominated by chiral extrapolation
uncertainties.Comment: Lattice2002(Heavy quark physics

### Lattice QCD on Small Computers

We demonstrate that lattice QCD calculations can be made $10^3$--$10^6$ times
faster by using very coarse lattices. To obtain accurate results, we replace
the standard lattice actions by perturbatively-improved actions with
tadpole-improved correction terms that remove the leading errors due to the
lattice. To illustrate the power of this approach, we calculate the
static-quark potential, and the charmonium spectrum and wavefunctions using a
desktop computer. We obtain accurate results that are independent of the
lattice spacing and agree well with experiment.Comment: 15 pages, 3 figs incl as LaTex pictures Minor additions to tables and
tex

### $B_c$ Spectroscopy from Lattice QCD

We present first results for $B_c$ spectroscopy using Lattice
Non-Relativistic QCD (NRQCD). For the NRQCD action the leading order
spin-dependent and next to leading order spin-independent interactions have
been included with tadpole-improved coefficients. We use multi-exponential fits
to multiple correlation functions to extract ground and excited $S$ states and
give accurate values for the $S$ state hyperfine splitting and the P state
($B^{**}_c$) fine structure, including the effects of $^1P_1/^3P_1$ mixing.Comment: 12 pages uuencoded latex file + 1 postscript figur

### Estimate of the hadronic vacuum polarization disconnected contribution to the anomalous magnetic moment of the muon from lattice QCD

The quark-line disconnected diagram is a potentially important ingredient in lattice QCD calculations of the hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon. It is also a notoriously difficult one to evaluate. Here, for the first time, we give an estimate of this contribution based on lattice QCD results that have a statistically significant signal, albeit at one value of the lattice spacing and an unphysically heavy value of the u/d quark mass. We use HPQCDâ€™s method of determining the anomalous magnetic moment by reconstructing the Adler function from time moments of the current-current correlator at zero spatial momentum. Our results lead to a total (including u, d and s quarks) quark-line disconnected contribution to aÎ¼ of âˆ’0.15% of the u/d hadronic vacuum polarization contribution with an uncertainty which is 1% of that contribution

- â€¦