1,862 research outputs found
Radiative Transitions in Charmonium from Lattice QCD
Radiative transitions between charmonium states offer an insight into the
internal structure of heavy-quark bound states within QCD. We compute, for the
first time within lattice QCD, the transition form-factors of various
multipolarities between the lightest few charmonium states. In addition, we
compute the experimentally unobservable, but physically interesting vector
form-factors of the and .
To this end we apply an ambitious combination of lattice techniques,
computing three-point functions with heavy domain wall fermions on an
anisotropic lattice within the quenched approximation. With an anisotropy
at we find a reasonable gross spectrum and a
hyperfine splitting , which compares favourably with
other improved actions.
In general, after extrapolation of lattice data at non-zero to the
photopoint, our results agree within errors with all well measured experimental
values. Furthermore, results are compared with the expectations of simple quark
models where we find that many features are in agreement; beyond this we
propose the possibility of constraining such models using our extracted values
of physically unobservable quantities such as the quadrupole moment.
We conclude that our methods are successful and propose to apply them to the
problem of radiative transitions involving hybrid mesons, with the eventual
goal of predicting hybrid meson photoproduction rates at the GlueX experiment.Comment: modified version as publishe
Calculation of the nucleon axial charge in lattice QCD
Protons and neutrons have a rich structure in terms of their constituents,
the quarks and gluons. Understanding this structure requires solving Quantum
Chromodynamics (QCD). However QCD is extremely complicated, so we must
numerically solve the equations of QCD using a method known as lattice QCD.
Here we describe a typical lattice QCD calculation by examining our recent
computation of the nucleon axial charge.Comment: Prepared for Scientific Discovery through Advanced Computing (SciDAC
2006), Denver, Colorado, June 25-29 200
Nucleon structure in the chiral regime with domain wall fermions on an improved staggered sea
Moments of unpolarized, helicity, and transversity distributions,
electromagnetic form factors, and generalized form factors of the nucleon are
presented from a preliminary analysis of lattice results using pion masses down
to 359 MeV. The twist two matrix elements are calculated using a mixed action
of domain wall valence quarks and asqtad staggered sea quarks and are
renormalized perturbatively. Several observables are extrapolated to the
physical limit using chiral perturbation theory. Results are compared with
experimental moments of quark distributions and electromagnetic form factors
and phenomenologically determined generalized form factors, and the
implications on the transverse structure and spin content of the nucleon are
discussed.Comment: Talks of J.W. Negele and D.B. Renner at Lattice 200
Baryon Operators and Baryon Spectroscopy
The issues involved in a determination of the baryon resonance spectrum in
lattice QCD are discussed. The variational method is introduced and the need to
construct a sufficient basis of interpolating operators is emphasised. The
construction of baryon operators using group-theory techniques is outlined. We
find that the use both of quark-field smearing and link-field smearing in the
operators is essential firstly to reduce the coupling of operators to
high-frequency modes and secondly to reduce the gauge-field fluctuations in
correlators. We conclude with a status report of our current investigation of
baryon spectroscopy.Comment: Invited talk at Workshop on Computational Hadron Physics, Cyprus,
Sept. 14-17, 200
Nucleon Generalized Parton Distributions from Full Lattice QCD
We present a comprehensive study of the lowest moments of nucleon generalized
parton distributions in N_f=2+1 lattice QCD using domain wall valence quarks
and improved staggered sea quarks. Our investigation includes helicity
dependent and independent generalized parton distributions for pion masses as
low as 350 MeV and volumes as large as (3.5 fm)^3, for a lattice spacing of
0.124 fm. We use perturbative renormalization at one-loop level with an
improvement based on the non-perturbative renormalization factor for the axial
vector current, and only connected diagrams are included in the isosinglet
channel.Comment: 40 pages, 49 figures; Revised chiral extrapolations in sections A-K,
main conclusions unchange
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