187 research outputs found
Strangeness in the Meson Cloud Model
I review progress in calculating strange quark and antiquark distributions of
the nucleon using the meson cloud model. This progress parallels that of the
meson cloud model, which is now a useful theoretical basis for understanding
symmetry breaking in nucleon parton distribution functions. I examine the
breaking of symmetries involving strange quarks and antiquarks, including quark
- antiquark symmetry in the sea, SU(3) flavour symmetry and SU(6) spin-flavour
symmetry.Comment: 6 pages, 4 figures. Talk given at workshop "Achievements and New
Directions in Subatomic Physics: Workshop in honour of Tony Thomas' 60th
birthday," CSSM, Adelaide, South Australia, 15 - 19 February 2010
g1(x) and g2(x) in the Meson Cloud Model
We calculate the spin dependent structure functions g1(x) and g2(x) of the
proton and neutron. Our calculation uses the meson cloud model of nucleon
structure and includes the effects of kinematic terms which mix transverse and
longitudinal spin components. We find small corrections to the nucleon
structure functions, however these are significant for the neutron.Comment: 4 pages, contribution to DIS 05, Madison, Wisconsi
The flavour asymmetry of polarized anti-quarks in the nucleon
We present a study of the flavour asymmetry of polarized anti-quarks in the
nucleon using the meson cloud model. We include contributions both from the
vector mesons and the interference terms of pseudoscalar and vector mesons.
Employing the bag model, we first give the polarized valence quark distribution
of the meson and the interference distributions. Our calculations show
that the interference effect mildly increases the prediction for \Delta
\dbar(x)-\Delta \ubar(x) at intermediate region. We also discuss the
contribution of `Pauli blocking' to the asymmetry.Comment: 22 pages, LaTex, 5 PS figures. Version to appear in Eur. Phys. J. C.
An appendix is added for expressions for the helicity dependent fluctuation
functions. An error in the programme for fluctuation function
f_{(\pi\rho)\Delta /N} is corrected, which increases numerical results by
about 10%. Unchanged conclusion
Comparison of gluon flux-tube distributions for quark-diquark and quark-antiquark hadrons
The distribution of gluon fields in hadrons is of fundamental interest in
QCD. Using lattice QCD we have observed the formation of gluon flux tubes
within 3 quark (baryon) and quark plus antiquark (meson) systems for a wide
variety of spatial distributions of the color sources. In particular we have
investigated three quark configurations where two of the quarks are close
together and the third quark is some distance away, which approximates a quark
plus diquark string. We find that the string tension of the quark-diquark
string is the same as that of the quark-antiquark string on the same lattice.
We also compare the longitudinal and transverse profiles of the gluon flux
tubes for both sets of strings, and find them to be of similar radii and to
have similar vacuum suppression.Comment: 6 pages, 9 figures and 1 tabl
Determining the strange and antistrange quark distributions of the nucleon
The difference between the strange and antistrange quark distributions,
\delta s(x)=s(x)-\sbar(x), and the combination of light quark sea and strange
quark sea, \Delta (x)=\dbar(x)+\ubar(x)-s(x)-\sbar(x), are originated from
non-perturbative processes, and can be calculated using non-perturbative models
of the nucleon. We report calculations of and using
the meson cloud model. Combining our calculations of with
relatively well known light antiquark distributions obtained from global
analysis of available experimental data, we estimate the total strange sea
distributions of the nucleon.Comment: 4 pages, 3 figures; talk given by F.-G. at QNP0
Gluon flux-tube distribution and linear confinement in baryons
We have observed the formation of gluon flux-tubes within baryons using
lattice QCD techniques. A high-statistics approach, based on translational and
rotational symmetries of the four-dimensional lattice, enables us to observe
correlations between vacuum action density and quark positions in a completely
gauge independent manner. This contrasts with earlier studies which used
gauge-dependent smoothing techniques. We used 200 O(a^2) improved quenched QCD
gauge-field configurations on a 16^3x32 lattice with a lattice spacing of 0.123
fm. In the presence of static quarks flux tubes representing the suppression of
gluon-field fluctuations are observed. We have analyzed 11 L-shapes and 8 T and
Y shapes of varying sizes in order to explore a variety of flux-tube
topologies, including the ground state. At large separations, Y-shape flux-tube
formation is observed. T-shaped paths are observed to relax towards a Y-shaped
topology, whereas L-shaped paths give rise to a large potential energy. We do
not find any evidence for the formation of a Delta-shaped flux-tube (empty
triangle) distribution. However, at small quark separations, we observe an
expulsion of gluon-field fluctuations in the shape of a filled triangle with
maximal expulsion at the centre of the triangle. Having identified the precise
geometry of the flux distribution, we are able to perform quantitative
comparison between the length of the flux-tube and the associated static quark
potential. For every source configuration considered we find a universal string
tension, and conclude that, for large quark separations, the ground state
potential is that which minimizes the length of the flux-tube. The flux tube
radius of the baryonic ground state potential is found to be 0.38 \pm 0.03 fm,
with vacuum fluctuations suppressed by 7.2 \pm 0.6 %.Comment: 16 pages, final version as accepted for publication in Physical
review D1. Abstract, text, references and some figures have been revise
The Flavor Asymmetry of the Nucleon Sea
We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon
sea arising from gluon exchange and pion exchange between confined quarks.
While the effect is primarily to suppress anti-down relative to anti-up quarks,
this is numerically insignificant for the pion terms.Comment: To appear in Phys. Rev.
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