2,056 research outputs found
Non-perturbative O(a) improvement of the vector current
We discuss non-perturbative improvement of the vector current, using the
Schroedinger Functional formalism. By considering a suitable Ward identity, we
compute the improvement coefficient which gives the O(a) mixing of the tensor
current with the vector one.Comment: 3 pages (LaTeX, 2 ps figures, styles), talk presented at Lattice 9
Non-perturbative renormalization of the axial current with improved Wilson quarks
We present a new normalization condition for the axial current, which is
derived from the PCAC relation with non-vanishing mass.
Using this condition reduces the O(r_0 m) corrections to the axial current
normalization constant Z_A for an easier chiral extrapolation in the cases,
where simulations at zero quark-mass are not possible. The method described
here also serves as a preparation for a determination of Z_A in the full
two-flavor theory.Comment: 3 pages, 3 figures, Lattice2003(improve
Non-Perturbative Improvement of the Anisotropic Wilson QCD Action
We describe the first steps in the extension of the Symanzik O()
improvement program for Wilson-type quark actions to anisotropic lattices, with
a temporal lattice spacing smaller than the spatial one. This provides a fully
relativistic and computationally efficient framework for the study of heavy
quarks. We illustrate our method with accurate results for the quenched
charmonium spectrum.Comment: LATTICE98(improvement), 3 pages, 4 figure
Moments of parton evolution probabilities on the lattice within the Schroedinger functional scheme
We define, within the Schroedinger functional scheme (SF), the matrix
elements of the twist-2 operators corresponding to the first two moments of
non-singlet parton densities. We perform a lattice one-loop calculation that
fixes the relation between the SF scheme and other common schemes and shows the
main source of lattice artefacts. This calculation sets the basis for a
numerical evaluation of the non-perturbative running of parton densities.Comment: Latex file, 4 figures, 15 page
A New Way to Set the Energy Scale in Lattice Gauge Theories and its Application to the Static Force and in SU(2) Yang--Mills Theory
We introduce a hadronic scale through the force between static
quarks at intermediate distances . The definition amounts
to ~fm in phenomenological potential models. Since is
well defined and can be calculated accurately in a Monte Carlo simulation, it
is an ideal quantity to set the scale. In SU(2) pure gauge theory, we use new
data (and to set the scale) to extrapolate to the continuum limit
for distances ~fm to ~fm. Through we determine the energy
scale in the recently calculated running coupling, which used the recursive
finite size technique to reach large energy scales. Also in this case, the
lattice data can be extrapolated to the continuum limit. The use of one loop
Symanzik improvement is seen to reduce the lattice spacing dependence
significantly.
Warning: The preprint is not completely fresh, but maybe you haven't seen
it...Comment: accepted in Nucl. Phys. B, 18 pages postscript-file with all figure
A perturbative determination of O(a) boundary improvement coefficients for the Schr\"odinger Functional coupling at 1-loop with improved gauge actions
We determine O() boundary improvement coefficients up to 1-loop level for
the Schr\"odinger Functional coupling with improved gauge actions including
plaquette and rectangle loops. These coefficients are required to implement
1-loop O() improvement in full QCD simulations for the coupling with the
improved gauge actions. To this order, lattice artifacts of step scaling
function for each improved gauge action are also investigated. In addition,
passing through the SF scheme, we estimate the ratio of -parameters
between the improved gauge actions and the plaquette action more accurately.Comment: 17 pages, 2 figures, 6 table
A new simulation algorithm for lattice QCD with dynamical quarks
A previously introduced multi-boson technique for the simulation of QCD with
dynamical quarks is described and some results of first test runs on a
lattice with Wilson quarks and gauge group SU(2) are reported.Comment: 7 pages, postscript file (166 KB
Lattice QCD without topology barriers
As the continuum limit is approached, lattice QCD simulations tend to get
trapped in the topological charge sectors of field space and may consequently
give biased results in practice. We propose to bypass this problem by imposing
open (Neumann) boundary conditions on the gauge field in the time direction.
The topological charge can then flow in and out of the lattice, while many
properties of the theory (the hadron spectrum, for example) are not affected.
Extensive simulations of the SU(3) gauge theory, using the HMC and the closely
related SMD algorithm, confirm the absence of topology barriers if these
boundary conditions are chosen. Moreover, the calculated autocorrelation times
are found to scale approximately like the square of the inverse lattice
spacing, thus supporting the conjecture that the HMC algorithm is in the
universality class of the Langevin equation.Comment: Plain TeX source, 26 pages, 4 figures include
The gradient flow running coupling with twisted boundary conditions
We study the gradient flow for Yang-Mills theories with twisted boundary
conditions. The perturbative behavior of the energy density is used to define a running coupling at a scale given by the
linear size of the finite volume box. We compute the non-perturbative running
of the pure gauge coupling constant and conclude that the technique is
well suited for further applications due to the relatively mild cutoff effects
of the step scaling function and the high numerical precision that can be
achieved in lattice simulations. We also comment on the inclusion of matter
fields.Comment: 27 pages. LaTe
Continuous external momenta in non-perturbative lattice simulations: a computation of renormalization factors
We discuss the usage of continuous external momenta for computing
renormalization factors as needed to renormalize operator matrix elements.
These kind of external momenta are encoded in special boundary conditions for
the fermion fields. The method allows to compute certain renormalization
factors on the lattice that would have been very difficult, if not impossible,
to compute with standard methods. As a result we give the renormalization group
invariant step scaling function for a twist-2 operator corresponding to the
average momentum of non-singlet quark densities.Comment: 28 pages, 10 figure
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