3,485 research outputs found
Moving NRQCD for B Form Factors at High Recoil
We derive the continuum and lattice tree-level moving NRQCD (mNRQCD) through
order 1/m^2. mNRQCD is a generalization of NRQCD for dealing with hadrons with
nonzero velocity u_mu. The quark's total momentum is written as P^mu=Mu^mu+k^mu
where k^mu << Mu^mu is discretized and Mu^mu is treated exactly. Radiative
corrections to couplings on the lattice are discussed. mNRQCD is particularly
useful for calculating B->pi and B->D form factors since errors are similar at
low and high recoil.Comment: 3 pages, 1 figure, Lattice2002(heavyquark
DEPENDENCE OF THE CURRENT RENORMALISATION CONSTANTS ON THE QUARK MASS
We study the behaviour of the vector and axial current renormalisation
constants and as a function of the quark mass, . We show that
sizeable and systematic effects are present in the
Wilson and Clover cases respectively. We find that the prescription of
Kronfeld, Lepage and Mackenzie for correcting these artefacts is not always
successful.Comment: Contribution to Lattice'94, 3 pages PostScript, uuencoded compressed
The D234 action for light quarks
We investigate a new light fermion action (the ``D234'' action), which is
accurate up to \O(a^3) and tadpole-improved \O(a \alpha_s) errors. Using
D234 with Symanzik- and tadpole-improved glue we find evidence that continuum
results for the quenched hadron spectrum (pion, rho and nucleon) can be
obtained on coarse lattices.Comment: Latex, 4 pages, submitted to Lattice '95 proceeding
Improving Lattice Quark Actions
We explore the first stage of the Symanzik improvement program for lattice
Dirac fermions, namely the construction of doubler-free, highly improved
classical actions on isotropic as well as anisotropic lattices (where the
temporal lattice spacing, a_t, is smaller than the spatial one). Using field
transformations to eliminate doublers, we derive the previously presented
isotropic D234 action with O(a^3) errors, as well as anisotropic D234 actions
with O(a^4) or O(a_t^3, a^4) errors. Besides allowing the simulation of heavy
quarks within a relativistic framework, anisotropic lattices alleviate
potential problems due to unphysical branches of the quark dispersion relation
(which are generic to improved actions), facilitate studies of lattice
thermodynamics, and allow accurate mass determinations for particles with bad
signal/noise properties, like glueballs and P-state mesons. We also show how
field transformations can be used to completely eliminate unphysical branches
of the dispersion relation. Finally, we briefly discuss future steps in the
improvement program.Comment: Tiny changes to agree with version to appear in Nucl. Phys. B (33
pages, LaTeX, 13 eps files
Improving lattice perturbation theory
Lepage and Mackenzie have shown that tadpole renormalization and systematic
improvement of lattice perturbation theory can lead to much improved numerical
results in lattice gauge theory. It is shown that lattice perturbation theory
using the Cayley parametrization of unitary matrices gives a simple analytical
approach to tadpole renormalization, and that the Cayley parametrization gives
lattice gauge potentials gauge transformations close to the continuum form. For
example, at the lowest order in perturbation theory, for SU(3) lattice gauge
theory, at the `tadpole renormalized' coupling to be compared to the non-perturbative numerical value Comment: Plain TeX, 8 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
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
Improved Nonrelativistic QCD for Heavy Quark Physics
We construct an improved version of nonrelativistic QCD for use in lattice
simulations of heavy quark physics, with the goal of reducing systematic errors
from all sources to below 10\%. We develop power counting rules to assess the
importance of the various operators in the action and compute all leading order
corrections required by relativity and finite lattice spacing. We discuss
radiative corrections to tree level coupling constants, presenting a procedure
that effectively resums the largest such corrections to all orders in
perturbation theory. Finally, we comment on the size of nonperturbative
contributions to the coupling constants.Comment: 40 pages, 2 figures (not included), in LaTe
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
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