62,533 research outputs found
Aspects of quark mass generation on a torus
In this talk we report on recent results for the quark propagator on a
compact manifold. The corresponding Dyson-Schwinger equations on a torus are
solved on volumes similar to the ones used in lattice calculations. The
quark-gluon interaction is fixed such that the lattice results are reproduced.
We discuss both the effects in the infinite volume/continuum limit as well as
effects when the volume is small.Comment: 3 pages, 3 figures; talk given by CF at QNP06, Madrid, June 200
Finite volume effects in a quenched lattice-QCD quark propagator
We investigate finite volume effects in the pattern of chiral symmetry
breaking. To this end we employ a formulation of the Schwinger-Dyson equations
on a torus which reproduces results from the corresponding lattice simulations
of staggered quarks and from the overlap action. Studying the volume dependence
of the quark propagator we find quantitative differences with the infinite
volume result at small momenta and small quark masses. We estimate the minimal
box length L below which chiral perturbation theory cannot be applied to be L
\simeq 1.6 fm. In the infinite volume limit we find a chiral condensate of
||_{\bar{MS}}^{2 GeV} = (253 \pm 5.0 MeV)^3, an up/down quark mass of
m_{\bar{MS}}^{2 GeV} = 4.1 \pm 0.3 MeV and a pion decay constant which is only
ten percent smaller than the experimental value.Comment: 19 pages, 8 figures. v2: minor clarifications added, version
published in PR
Rayleigh-Benard Convection with a Radial Ramp in Plate Separation
Pattern formation in Rayleigh-Benard convection in a large-aspect-ratio
cylinder with a radial ramp in the plate separation is studied analytically and
numerically by performing numerical simulations of the Boussinesq equations. A
horizontal mean flow and a vertical large scale counterflow are quantified and
used to understand the pattern wavenumber. Our results suggest that the mean
flow, generated by amplitude gradients, plays an important role in the roll
compression observed as the control parameter is increased. Near threshold the
mean flow has a quadrupole dependence with a single vortex in each quadrant
while away from threshold the mean flow exhibits an octupole dependence with a
counter-rotating pair of vortices in each quadrant. This is confirmed
analytically using the amplitude equation and Cross-Newell mean flow equation.
By performing numerical experiments the large scale counterflow is also found
to aid in the roll compression away from threshold but to a much lesser degree.
Our results yield an understanding of the pattern wavenumbers observed in
experiment away from threshold and suggest that near threshold the mean flow
and large scale counterflow are not responsible for the observed shift to
smaller than critical wavenumbers.Comment: 10 pages, 13 figure
Observation of the Quantum Zeno and Anti-Zeno effects in an unstable system
We report the first observation of the Quantum Zeno and Anti-Zeno effects in
an unstable system. Cold sodium atoms are trapped in a far-detuned standing
wave of light that is accelerated for a controlled duration. For a large
acceleration the atoms can escape the trapping potential via tunneling.
Initially the number of trapped atoms shows strong non-exponential decay
features, evolving into the characteristic exponential decay behavior. We
repeatedly measure the number of atoms remaining trapped during the initial
period of non-exponential decay. Depending on the frequency of measurements we
observe a decay that is suppressed or enhanced as compared to the unperturbed
system.Comment: 4 pages, 5 figures, submitted to PR
A general review of concepts for reducing skin friction, including recommendations for future studies
Four main concepts which have significantly reduced skin friction in experimental studies are discussed; suction, gaseous injection, particle additives, and compliant wall. It is considered possible that each of these concepts could be developed and applied in viable skin friction reduction systems for aircraft application. Problem areas with each concept are discussed, and recommendations for future studies are made
Form Factors of Baryons in a Confining and Covariant Diquark-Quark Model
We treat baryons as bound states of scalar or axialvector diquarks and a
constituent quark which interact through quark exchange. This description
results as an approximation to the relativistic Faddeev equation for three
quarks which yields an effective Bethe-Salpeter equation. Octet and decuplet
masses and fully four-dimensional wave functions have been computed for two
cases: assuming an essentially pointlike diquark on the one hand, and a diquark
with internal structure on the other hand. Whereas the differences in the mass
spectrum are fairly small, the nucleon electromagnetic form factors are greatly
improved assuming a diquark with structure. First calculations to the
pion-nucleon form factor also suggest improvements.Comment: 11 pages, uses 'aipproc.sty'. Talk given by M.O. at the Workshop
"Effective Theories of Low Energy QCD", Coimbra, Portugal, Sep 10-15 199
Quark Condensates: Flavour Dependence
We determine the q-bar q condensate for quark masses from zero up to that of
the strange quark within a phenomenologically successful modelling of continuum
QCD by solving the quark Schwinger-Dyson equation. The existence of multiple
solutions to this equation is the key to an accurate and reliable extraction of
this condensate using the operator product expansion. We explain why
alternative definitions fail to give the physical condensate.Comment: 9 pages, 7 figures, uses appolb.cls, LaTeX. Talk presented by R.
Williams at the EURIDICE Final Meeting, August 24-27th, 2006, Kazimierz,
Polan
- …