174 research outputs found
Incorporating Chiral Symmetry in Extrapolations of Octet Baryon Magnetic Moments
We explore methods of extrapolating lattice calculations of hadronic
observables to the physical regime, while respecting the constraints of chiral
symmetry and heavy quark effective theory. In particular, we extrapolate
lattice results for magnetic moments of the spin-1/2 baryon octet to the
physical pion mass and compare with experimental measurements. The success
previously reported for extrapolations of the nucleon magnetic moments carries
over to the Sigma baryons. A study of the residual discrepancies in the Xi
baryon moments suggests that it is important to have new simulation data with a
more realistic strange quark mass.Comment: 9 pages, 4 figure
Chiral Analysis of Quenched Baryon Masses
We extend to quenched QCD an earlier investigation of the chiral structure of
the masses of the nucleon and the delta in lattice simulations of full QCD.
Even after including the meson-loop self-energies which give rise to the
leading and next-to-leading non-analytic behaviour (and hence the most rapid
variation in the region of light quark mass), we find surprisingly little
curvature in the quenched case. Replacing these meson-loop self-energies by the
corresponding terms in full QCD yields a remarkable level of agreement with the
results of the full QCD simulations. This comparison leads to a very good
understanding of the origins of the mass splitting between these baryons.Comment: 23 pages, 6 figure
Chiral Behaviour of the Rho Meson in Lattice QCD
In order to guide the extrapolation of the mass of the rho meson calculated
in lattice QCD with dynamical fermions, we study the contributions to its
self-energy which vary most rapidly as the quark mass approaches zero; from the
processes and . It turns out that in
analysing the most recent data from CP-PACS it is crucial to estimate the
self-energy from using the same grid of discrete momenta as
included implicitly in the lattice simulation. The correction associated with
the continuum, infinite volume limit can then be found by calculating the
corresponding integrals exactly. Our error analysis suggests that a factor of
10 improvement in statistics at the lowest quark mass for which data currently
exists would allow one to determine the physical rho mass to within 5%.
Finally, our analysis throws new light on a long-standing problem with the
J-parameter.Comment: 13 pages, 7 figures. Full analytic forms of the self-energies are
included and a correction in the omega-pi self-energ
Quark contributions to baryon magnetic moments in full, quenched, and partially quenched QCD
The chiral nonanalytic behavior of quark-flavor contributions to the magnetic moments of octet baryons is determined in full, quenched and partially quenched QCD, using an intuitive and efficient diagrammatic formulation of quenched and partially quenched chiral perturbation theory. The technique provides a separation of quark-sector magnetic-moment contributions into direct sea-quark loop, valence-quark, indirect sea-quark loop and quenched valence contributions, the latter being the conventional view of the quenched approximation. Both meson and baryon mass violations of SU(3)-flavor symmetry are accounted for. Following a comprehensive examination of the individual quark-sector contributions to octet baryon magnetic moments, numerous opportunities to observe and test the underlying structure of baryons and the nature of chiral nonanalytic behavior in QCD and its quenched variants are discussed. In particular, the valence u-quark contribution to the proton magnetic moment provides the optimal opportunity to directly view nonanalytic behavior associated with the meson cloud of full QCD and the quenched meson cloud of quenched QCD. The u quark in Σ+ provides the best opportunity to display the artifacts of the quenched approximation.Derek B. Leinwebe
Systematic uncertainties in the precise determination of the strangeness magnetic moment of the nucleon
Systematic uncertainties in the recent precise determination of the
strangeness magnetic moment of the nucleon are identified and quantified. In
summary, G_M^s = -0.046 \pm 0.019 \mu_N.Comment: Invited presentation at PAVI '04, International Workshop on Parity
Violation and Hadronic Structure, Laboratoire de Physique Subatomique et de
Cosmologie, Grenoble, France, June 8-11, 2004. 7 pages, 16 figure
N* Masses from an Anisotropic Lattice QCD Action
We report N* masses in the spin 3/2 sector from a highly-improved anisotropic
action. States with both positive and negative parity are isolated via a parity
projection method. The extent to which spin projection is needed is examined.
The gross features of the splittings from the nucleon ground state show a trend
consistent with experimental results at the quark masses explored.Comment: Lattice2001(spectrum), 3 pages, 4 figures, new interpolating fiel
Electromagnetic Form Factors with FLIC fermions
The Fat-Link Irrelevant Clover (FLIC) fermion action provides a new form of
nonperturbative O(a) improvement and allows efficient access to the light
quark-mass regime. FLIC fermions enable the construction of the
nonperturbatively O(a)-improved conserved vector current without the
difficulties associated with the fine tuning of the improvement coefficients.
The simulations are performed with an O(a^2) mean-field improved
plaquette-plus-rectangle gluon action on a 20^3 x 40 lattice with a lattice
spacing of 0.128 fm, enabling the first simulation of baryon form factors at
light quark masses on a large volume lattice.
Magnetic moments, electric charge radii and magnetic radii are extracted from
these form factors, and show interesting chiral nonanalytic behavior in the
light quark mass regime.Comment: Presented by J.Zanotti at the Workshop on Lattice Hadron Physics,
Cairns, Australia, 2003. 7pp, 8 figure
Improved Landau Gauge Fixing and Discretisation Errors
Lattice discretisation errors in the Landau gauge condition are examined. An
improved gauge fixing algorithm in which order a^2 errors are removed is
presented. Order a^2 improvement of the gauge fixing condition displays the
secondary benefit of reducing the size of higher-order errors. These results
emphasise the importance of implementing an improved gauge fixing condition.Comment: LATTICE99 (Improvement and Renormalization), 3 pages, 1 figur
Hadron Mass Extraction from Lattice QCD
The extraction of quantities from lattice QCD calculations at realistic quark
masses is of considerable importance. Whilst physical quark masses are some way
off, the recent advances in the calculation of hadron masses within full QCD
now invite improved extrapolation methods. We show that, provided the correct
chiral behaviour of QCD is respected in the extrapolation to realistic quark
masses, one can indeed obtain a fairly reliable determination of masses, the
sigma commutator and the J parameter. We summarise these findings by presenting
the nonanalytic behaviour of nucleon and rho masses in the standard Edinburgh
plot.Comment: Talk presented by S. V. Wright at the Workshop on Lattice Hadron
Physics (LHP2001), Cairns, Australia, 9-18 July 2001, 8 pages, requires
espcrc2.sty (included
Excited Baryons from the FLIC Fermion Action
Masses of positive and negative parity excited nucleons and hyperons are
calculated in quenched lattice QCD using an O(a^2) improved gluon action and a
fat-link clover fermion action in which only the irrelevant operators are
constructed with fat links. The results are in agreement with earlier N*
simulations with improved actions, and exhibit a clear mass splitting between
the nucleon and its parity partner, as well as a small mass splitting between
the two low-lying J^P={1/2}^- N* states. Study of different Lambda
interpolating fields suggests a similar splitting between the lowest two
{1/2}^- Lambda* states, although the empirical mass suppression of the
Lambda*(1405) is not seen.Comment: 3 pages, 3 figures, Lattice2002(QCD Spectrum and Quark Masses
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