279 research outputs found
Moments of Isovector Quark Distributions in Lattice QCD
We investigate the connection of lattice calculations of moments of isovector
parton distributions to the physical regime through extrapolations in the quark
mass. We consider the one pion loop renormalisation of the nucleon matrix
elements of the corresponding operators and thereby develop formulae with which
to extrapolate the moments of the unpolarised, helicity and transversity
distributions. These formulae are consistent with chiral perturbation theory in
the chiral limit and incorporate the correct heavy quark limits. In the
polarised cases, the inclusion of intermediate states involving the
isobar is found to be very important. The results of our extrapolations are in
general agreement with the phenomenological values of these moments where they
are known, and for the first time we perform an extrapolation of the low
moments of the isovector transversity distribution which is consistent with
chiral symmetry.Comment: 3 pages, 3 figures. Talk given by W. Detmold. at QNP200
Parton distributions from lattice QCD
We extract the x dependence of the nonsinglet u-d distribution function in
the nucleon from the lowest few moments calculated on the lattice, using an
extrapolation formula which ensures the correct behavior in the chiral and
heavy quark limits. We discuss the implications for the quark mass dependence
of meson masses lying on the J(PC)=1(--) Regge trajectory.Comment: 18 pages, 6 figures, minor changes made for publicatio
Connecting structure functions on the lattice with phenomenology
We examine the extraction of moments of parton distributions from lattice
data, focusing in particular on the chiral extrapolation as a function of the
quark mass. Inclusion of the correct chiral behavior of the spin-averaged
isovector distribution resolves a long-standing discrepancy between the lattice
moments and experiment. We extract the x-dependence of the valence u-d
distribution from the lowest few lattice moments, and discuss the implications
for the quark mass dependence of meson masses lying on the rho Regge
trajectory. The role of chiral symmetry in spin-dependent distributions, and in
particular the lattice axial vector charge, g_A, is also highlighted.Comment: 8 pages, 5 figures, plenary talk given by W. Melnitchouk at the 3rd
Circum-Pan-Pacific Symposium on High Energy Spin Physics (Beijing, China,
October 2001), to appear in Int. J. Mod. Phys.
Study of Lattice QCD Form Factors Using the Extended Gari-Krumpelmann Model
We explore the suitability of a modern vector meson dominance (VMD) model as
a method for chiral extrapolation of nucleon electromagnetic form factor
simulations in lattice QCD. It is found that the VMD fits to experimental data
can be readily generalized to describe the lattice simulations. However, the
converse is not true. That is, the VMD form is unsuitable as a method of
extrapolation of lattice simulations at large quark mass to the physical
regime.Comment: 14 pages, 5 figure
Electromagnetic Polarizabilities: Lattice QCD in Background Fields
Chiral perturbation theory makes definitive predictions for the extrinsic
behavior of hadrons in external electric and magnetic fields. Near the chiral
limit, the electric and magnetic polarizabilities of pions, kaons, and nucleons
are determined in terms of a few well-known parameters. In this limit, hadrons
become quantum mechanically diffuse as polarizabilities scale with the inverse
square-root of the quark mass. In some cases, however, such predictions from
chiral perturbation theory have not compared well with experimental data.
Ultimately we must turn to first principles numerical simulations of QCD to
determine properties of hadrons, and confront the predictions of chiral
perturbation theory. To address the electromagnetic polarizabilities, we
utilize the background field technique. Restricting our attention to
calculations in background electric fields, we demonstrate new techniques to
determine electric polarizabilities and baryon magnetic moments for both
charged and neutral states. As we can study the quark mass dependence of
observables with lattice QCD, the lattice will provide a crucial test of our
understanding of low-energy QCD, which will be timely in light of ongoing
experiments, such as at COMPASS and HI\gamma S.Comment: 3 pages, talk given by B. C. Tiburzi at PANIC 201
Analytic structure of the Landau gauge gluon propagator
The results of different non-perturbative studies agree on a power law as the
infrared behavior of the Landau gauge gluon propagator. This propagator
violates positivity and thus indicates the absence of the transverse gluons
from the physical spectrum, i.e. gluon confinement. A simple analytic structure
for the gluon propagator is proposed capturing all of its features. We comment
also on related investigations for the Landau gauge quark propagator.Comment: 4 pages, 2 figures, talk given by R.A. at 6th Conference on Quark
Confinement and the Hadron Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep
200
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