235 research outputs found
Nucleon form factors and moments of parton distributions in twisted mass lattice QCD
We present results on the electroweak form factors and on the lower moments
of parton distributions of the nucleon, within lattice QCD using two dynamical
flavors of degenerate twisted mass fermions. Results are obtained on lattices
with three different values of the lattice spacings, namely a=0.089 fm, a=0.070
fm and a=0.056 fm, allowing the investigation of cut-off effects. The volume
dependence is examined by comparing results on two lattices of spatial length
L=2.1 fm and L=2.8 fm. The simulations span pion masses in the range of 260-470
MeV. Our results are renormalized non-perturbatively and the values are given
in the MS-scheme at a scale mu=2 GeV.Comment: Talk presented in the XXIst International Europhysics Conference on
High Energy Physics, 21-27 July 2011, Grenoble, Rhones Alpes Franc
Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon
A dispersion relation formalism for the virtual Compton scattering (VCS)
reaction on the proton is presented, which for the first time allows a
dispersive evaluation of 4 generalized polarizabilities at a four-momentum
transfer 0.5 GeV. The dispersive integrals are calculated using
a state-of-the-art pion photo- and electroproduction analysis. The dispersion
formalism provides a new tool to analyze VCS experiments above pion threshold,
thus increasing the sensitivity to the generalized polarizabilities of the
nucleon.Comment: 4 pages, 2 figure
Quark-meson coupling model with the cloudy bag
Using the volume coupling version of the cloudy bag model, the quark-meson
coupling model is extended to study the role of pion field and the properties
of nuclear matter. The extended model includes the effect of gluon exchange as
well as the pion-cloud effect, and provides a good description of the nuclear
matter properties. The relationship between the extended model and the EFT
approach to nuclear matter is also discussed.Comment: 13 pages, 3 figure
Comparative efficacy of two Ivermectin Pour-on anthelmintics in beef steers in a commercial feedyard
Generic products generally have a cost advantage for beef producers over brand-name products. Recently, many beef producers have debated whether to utilize generic anthelmintics in cow/calf herds and feeder cattle. If generics are to be justified, the products must be proven to have efficacy similar to the brand-name product. Previous studies have indicated that generic macrocyclic lactones are less effective in controlling gastrointestinal parasites of cattle than the original brand-name products. The objective of this study was to compare the efficacy of Vetrimec (Norbrook Laboratories Limited, Newry, Co. Down, Northern Ireland) pour-on and Ivomec (Merial Animal Health, Duluth, GA) pour-on by utilizing the fecal egg reduction test in newly arrived feedlot steers
Scalar susceptibility and chiral symmetry restoration in nuclei
We study the nuclear modification of the scalar QCD susceptibility,
calculated as the derivative of the quark condensate with respect to the quark
mass. We show that it has two origins. One is the low lying nuclear
excitations. At normal nuclear density this part is constrained by the nuclear
incompressibility. The other part arises from the individual nucleon response
and it is dominated by the pion cloud contribution. Numerically the first
contribution dominates. The resulting increase in magnitude of the scalar
susceptibility at normal density is such that it becomes close to the
pseudoscalar susceptibility, while it is quite different in the vacuum. We
interpret it as a consequence of chiral symmetry restoration in nuclei.Comment: 17 pages, 5 figure
Chiral symmetry and quantum hadro-dynamics
Using the linear sigma model, we study the evolutions of the quark condensate
and of the nucleon mass in the nuclear medium. Our formulation of the model
allows the inclusion of both pion and scalar-isoscalar degrees of freedom. It
guarantees that the low energy theorems and the constrains of chiral
perturbation theory are respected. We show how this formalism incorporates
quantum hadro-dynamics improved by the pion loops effects.Comment: 24 pages, 2 figure
Low-energy and low-momentum representation of the virtual Compton scattering amplitude
We perform an expansion of the virtual Compton scattering amplitude for low
energies and low momenta and show that this expansion covers the transition
from the regime to be investigated in the scheduled photon electroproduction
experiments to the real Compton scattering regime.
We discuss the relation of the generalized polarizabilities of virtual
Compton scattering to the polarizabilities of real Compton scattering.Comment: 13 pages, LaTeX2e/RevTeX, no figure
Nucleon generalized polarizabilities within a relativistic Constituent Quark Model
Nucleon generalized polarizabilities are investigated within a relativistic
framework, defining such quantities through a Lorentz covariant multipole
expansion of the amplitude for virtual Compton scattering. The key physical
ingredients in the calculation of the nucleon polarizabilities are the Lorentz
invariant reduced matrix elements of the electromagnetic transition current,
which can be evaluated from off-energy-shell helicity amplitudes. The evolution
of the proton paramagnetic polarizability, , as a function of
the virtual-photon three-momentum transfer is explicitly evaluated within
a relativistic constituent quark model by adopting transition form factors
obtained in the light-front formalism. The discussion is focussed on the role
played by the effects due to the relativistic approach and to the transition
form factors, derived within different models.Comment: 14 pages and three figures (included), to appear in Phys. Rev. C (May
1998
Variations of Hadron Masses and Matter Properties in Dense Nuclear Matter
Using a self-consistent quark model for nuclear matter we investigate
variations of the masses of the non-strange vector mesons, the hyperons and the
nucleon in dense nuclear matter (up to four times the normal nuclear density).
We find that the changes in the hadron masses can be described in terms of the
value of the scalar mean-field in matter. The model is then used to calculate
the density dependence of the quark condensate in-medium, which turns out to be
well approximated by a linear function of the nuclear density. Some relations
among the hadron properties and the in-medium quark condensate are discussed.Comment: 22 pages, University of Adelaide preperint ADP-94-20/T160, submitted
to Physical Review
Magnetic moment of hyperons in nuclear matter by using quark-meson coupling models
We calculate the magnetic moments of hyperons in dense nuclear matter by
using relativistic quark models. Hyperons are treated as MIT bags, and the
interactions are considered to be mediated by the exchange of scalar and vector
mesons which are approximated as mean fields. Model dependence is investigated
by using the quark-meson coupling model and the modified quark-meson coupling
model; in the former the bag constant is independent of density and in the
latter it depends on density. Both models give us the magnitudes of the
magnetic moments increasing with density for most octet baryons. But there is a
considerable model dependence in the values of the magnetic moments in dense
medium. The magnetic moments at the nuclear saturation density calculated by
the quark meson coupling model are only a few percents larger than those in
free space, but the magnetic moments from the modified quark meson coupling
model increase more than 10% for most hyperons. The correlations between the
bag radius of hyperons and the magnetic moments of hyperons in dense matter are
discussed.Comment: substantial changes in the text, submitted to PL
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