818 research outputs found
Strange quarks and lattice QCD
The last few years have seen a dramatic improvement in our knowledge of the
strange form factors of the nucleon. With regard to the vector from factors the
level of agreement between theory and experiment gives us considerable
confidence in our ability to calculate with non-perturbative QCD. The
calculation of the strange scalar form factor has moved significantly in the
last two years, with the application of new techniques which yield values
considerably smaller than believed for the past 20 years. These new values turn
out to have important consequences for the detection of neutralinos, a
favourite dark matter candidate. Finally, very recent lattice studies have
resurrected interest in the famed H-dibaryon, with modern chiral extrapolation
of lattice data suggesting that it may be only slightly unbound. We review some
of the major sources of uncertainty in that chiral extrapolation.Comment: Invited talk at the Asia-Pacific few Body Conference, Seoul Kore
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
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
Limitations of the heavy-baryon expansion as revealed by a pion-mass dispersion relation
The chiral expansion of nucleon properties such as mass, magnetic moment, and
magnetic polarizability are investigated in the framework of chiral
perturbation theory, with and without the heavy-baryon expansion. The analysis
makes use of a pion-mass dispersion relation, which is shown to hold in both
frameworks. The dispersion relation allows an ultraviolet cutoff to be
implemented without compromising the symmetries. After renormalization, the
leading-order heavy-baryon loops demonstrate a stronger dependence on the
cutoff scale, which results in weakened convergence of the expansion. This
conclusion is tested against the recent results of lattice quantum
chromodynamics simulations for nucleon mass and isovector magnetic moment. In
the case of the polarizability, the situation is even more dramatic as the
heavy-baryon expansion is unable to reproduce large soft contributions to this
quantity. Clearly, the heavy-baryon expansion is not suitable for every
quantity.Comment: Accepted for publication in EPJ C. Made changes based on referee
comments: clarifying sentences to conclusion 1. of Section IV, beginning of
Section V, and new footnote in Section VI, page 8. Added more detailed
explanation in paragraph 4 of Section III. Added citations of Phys.Rev. D60,
034014, and Phys.Lett. B716, 33
Chiral Extrapolation of Lattice Data for Heavy Baryons
The masses of heavy baryons containing a b quark have been calculated
numerically in lattice QCD with pion masses which are much larger than its
physical value. In the present work we extrapolate these lattice data to the
physical mass of the pion by applying the effective chiral Lagrangian for heavy
baryons, which is invariant under chiral symmetry when the light quark masses
go to zero and heavy quark symmetry when the heavy quark masses go to infinity.
A phenomenological functional form with three parameters, which has the correct
behavior in the chiral limit and appropriate behavior when the pion mass is
large, is proposed to extrapolate the lattice data. It is found that the
extrapolation deviates noticably from the naive linear extrapolation when the
pion mass is smaller than about 500MeV. The mass differences between Sigma_b
and Sigma_b^* and between Sigma_b^{(*)} and Lambda_b are also presented.
Uncertainties arising from both lattice data and our model parameters are
discussed in detail. We also give a comparision of the results in our model
with those obtained in the naive linear extrapolations.Comment: 29 pages, 9 figure
Expression analysis of low temperature-induced genes in wheat
Non-Peer ReviewedWheat (Triticum aestivum L.) is a widely adapted, economically important crop exhibiting winter, spring and intermediate growth habits. Winter wheat is seeded in the fall, over-winters, resumes growth in spring and is harvested in early summer. It also requires a period of low temperature (LT) exposure, experienced during the fall, to switch from the vegetative to reproductive phase in spring, a process known as vernalization. Low temperature also allows the wheat plant to cold-acclimate to withstand freezing winter temperatures. There has always been an interest to grow winter wheat because of its yield advantage over spring wheat. However, LT tolerance needs to be improved to prevent winter kill and maximize its yield potential. To achieve this more detailed understanding of molecular mechanisms underlying LT tolerance is required. Thus, objectives of this study were to determine the expression of a LT-induced gene and cDNA-AFLP profile in leaf and crown tissues of LT-exposed wheat plants. Survival of crown tissues after exposure to sub-zero temperatures is an indication of the level of LT tolerance of a cultivar. Thus, pattern and levels of expression of LT-induced genes and identification of LT-induced transcripts in this tissue will add to understanding of LT tolerance. Genotypes used in this study included a winter hardy cultivar, Norstar, a tender spring cultivar, Manitou and two-near-isogenic lines with the Vrn-A1 (spring Norstar) and vrn-A1 (winter Manitou) alleles of Manitou and Norstar, respectively. The dominant Vrn-A1 locus confers spring habit and therefore no requirement for vernalization. Quantitative real-time polymerase chain reaction (QPCR) for the cold-regulated gene, Wcor410, indicated that in leaf tissue the Vrn-A1 locus determined level of expression, being higher in the lines having the recessive vrn-A1 allele compared to the dominant Vrn-A1 allele lines. In the crown tissue, the Norstar genetic background led to the higher level of expression than in the Manitou background. cDNA-AFLP analysis also exhibited variable profiles between the two tissues
Spin-3/2 Nucleon and Delta Baryons in Lattice QCD
We present first results for masses of spin-3/2 N and Delta baryons in
lattice QCD using Fat-Link Irrelevant Clover (FLIC) fermions. Spin-3/2
interpolating fields providing overlap with both spin-3/2 and spin-1/2 states
are considered. In the isospin-1/2 sector, we observe, after appropriate spin
and parity projection, a strong signal for the J^P=3/2^- state together with a
weak but discernible signal for the 3/2^+ state with a mass splitting near that
observed experimentally. We also find good agreement between the 1/2^+/- masses
and earlier nucleon mass simulations with the standard spin-1/2 interpolating
field. For the isospin-3/2 Delta states, clear mass splittings are observed
between the various 1/2^+/- and 3/2^+/- channels, with the calculated level
orderings in good agreement with those observed empirically.Comment: 17 pages, 8 figures, 2 table
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 ÎŁ+ provides the best opportunity to display the artifacts of the quenched approximation.Derek B. Leinwebe
Convergence of chiral effective field theory
We formulate the expansion for the mass of the nucleon as a function of pion
mass within chiral perturbation theory using a number of different ultra-violet
regularisation schemes; including dimensional regularisation and various
finite-ranged regulators. Leading and next-to-leading order non-analytic
contributions are included through the standard one-loop Feynman graphs. In
addition to the physical nucleon mass, the expansion is constrained by recent,
extremely accurate, lattice QCD data obtained with two flavors of dynamical
quarks. The extent to which different regulators can describe the chiral
expansion is examined, while varying the range of quark mass over which the
expansions are matched. Renormalised chiral expansion parameters are recovered
from each regularisation prescription and compared. We find that the
finite-range regulators produce consistent, model-independent results over a
wide range of quark mass sufficient to solve the chiral extrapolation problem
in lattice QCD.Comment: 13 pages, 13 figures; To appear in Progress in Particle and Nuclear
Physics; presented at Erice School on Quarks in Hadrons and Nuclei, September
200
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
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