1,463 research outputs found
Nonperturbative ``Lattice Perturbation Theory''
We discuss a program for replacing standard perturbative methods with Monte
Carlo simulations in short distance lattice gauge theory calculations.Comment: 3 pages, uuencoded Latex file, two embedded figures and .sty file
include
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
Like-Sign W Boson Production at the LHC as a Probe of Double Parton Scattering
Double parton scattering, i.e. two parton hard scattering processes in the
same hadron-hadron collision, may constitute an important background for Higgs
and other new particle searches at the LHC. We point out that like-sign W pair
production provides a relatively clean way of searching for, and calibrating,
double parton scattering at the LHC.Comment: 10 pages, LaTeX, 5 figures, epsfi
On the fourth root prescription for dynamical staggered fermions
With the aim of resolving theoretical issues associated with the fourth root
prescription for dynamical staggered fermions in Lattice QCD simulations, we
consider the problem of finding a viable lattice Dirac operator D such that
(det D_{staggered})^{1/4} = det D. Working in the flavour field representation
we show that in the free field case there is a simple and natural candidate D
satisfying this relation, and we show that it has acceptable locality behavior:
exponentially local with localisation range vanishing ~ (a/m)^{1/2} for lattice
spacing a -> 0. Prospects for the interacting case are also discussed, although
we do not solve this case here.Comment: 29 pages, 2 figures; some revision and streamlining of the
discussions; results unchanged; to appear in PR
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
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
The interplay of soft and hard contributions in the electromagnetic pion form factor
We consider various relativistic models for the valence Fock-state wave
function of the pion. These models are obtained from simple instant-form wave
functions by applying a Melosh rotation to the spin part and by imposing
physical constraints on the parameters. We discuss how the soft and the hard
(perturbative) parts of the electromagnetic form factor are affected by the
choice of the model and by the Melosh rotation.Comment: 3 pages, 2 eps-figures, uses espcrc2.st
Sign problems, noise, and chiral symmetry breaking in a QCD-like theory
The Nambu-Jona-Lasinio model reduced to 2+1 dimensions has two different path
integral formulations: at finite chemical potential one formulation has a
severe sign problem similar to that found in QCD, while the other does not. At
large N, where N is the number of flavors, one can compute the probability
distributions of fermion correlators analytically in both formulations. In the
former case one finds a broad distribution with small mean; in the latter one
finds a heavy tailed positive distribution amenable to the cumulant expansion
techniques developed in earlier work. We speculate on the implications of this
model for QCD.Comment: 16 pages, 5 figures; Published version with minor changes from the
origina
- …