418 research outputs found
Topological Susceptibility on Dynamical Staggered Fermion Configurations
The topological susceptibility is one of the few physical quantities that
directly measure the properties of the QCD vacuum. Chiral perturbation theory
predicts that in the small quark mass limit the topological susceptibility
depends quadratically on the pion mass, approaching zero in the chiral limit.
Lattice calculations have difficulty reproducing this behavior. In this paper
we study the topological susceptibility on dynamical staggered fermion
configurations. Our results indicate that the lattice spacing has to be small,
around a~0.1fm for thin link staggered fermion actions to show the expected
chiral behavior. Our preliminary result indicates that fat link fermions, on
the other hand, reproduce the theoretical expectations even on lattices with
a~0.17fm. We argue that this is due to the improved flavor symmetry of fat link
fermionic actions.Comment: 16 pages, 4 figure
Acute anti-allodynic action of gabapentin in dorsal horn and primary somatosensory cortex: Correlation of behavioural and physiological data
Neuropathic pain is a debilitating consequence of neuronal injury or disease. Although first line treatments
include the alpha-2-delta (a2d)-ligands, pregabalin and gabapentin (GBP), the mechanism of their
anti-allodynic action is poorly understood. One specific paradox is that GBP relieves signs of neuropathic
pain in animal models within 30min of an intraperitoneal (IP) injection yet its actions in vitro on spinal
dorsal horn or primary afferent neurons take hours to develop. We found, using confocal Ca2þ imaging,
that substantia gelatinosa neurons obtained ex vivo from rats subjected to sciatic chronic constriction
injury (CCI) were more excitable than controls. We confirmed that GBP (100 mg/kg) attenuated mechanical
allodynia in animals subject to CCI within 30min of IP injection.Substantia gelatinosa neurons
obtained ex vivo from these animals no longer displayed CCI-induced increased excitability.
Electrophysiological analysis of substantia gelatinosa neurons ex vivo suggest that rapidly developing
in vivo anti-allodynic effects of GBP i) are mediated intracellularly, ii) involve actions on the neurotransmitter
release machinery and iii) depend on decreased excitatory synaptic drive to excitatory
neurons without major actions on inhibitory neurons or on intrinsic neuronal excitability. Experiments
using in vivo Ca2þ imaging showed that 100 mg/kg GBP also suppressed the response of the S1 somatosensory
cortex of CCI rats, but not that of control rats, to vibrotactile stimulation.
Since the level of a2d1 protein is increased in primary afferent fibres after sciatic CCI, we suggest this
dictates the rate of GBP action; rapidly developing actions can only be seen when a2d1 levels are
elevated
The strong coupling constant at small momentum as an instanton detector
We present a study of at small p computed from the lattice.
It shows a dramatic law which can be understood within an
instanton liquid model. In this framework the prefactor gives a direct measure
of the instanton density in thermalised configurations. A preliminary result
for this density is 5.27(4) fm^{-4}.Comment: 12 pages, 4 figure
Topological Structure of the SU(3) Vacuum
We investigate the topological structure of the vacuum in SU(3) lattice gauge
theory. We use under-relaxed cooling to remove the high-frequency fluctuations
and a variety of "filters" to identify the topological charges in the resulting
smoothened field configurations. We find a densely packed vacuum with an
average instanton size, in the continuum limit, of about 0.5 fm. The density at
large sizes decreases as a large inverse power of the size. At small sizes we
see some sign of a trend towards the asymptotic perturbative behaviour. We find
that an interesting polarisation phenomenon occurs: the large topological
charges tend to have, on the average, the same sign and are over-screened by
the smaller charges which tend to have, again on the average, the opposite sign
to the larger instantons. We also calculate the topological susceptibility for
which we obtain a continuum value of about 187 MeV. We perform the calculations
for various volumes, lattice spacings and numbers of cooling sweeps, so as to
obtain some control over the associated systematic errors. The coupling range
is from beta=6.0 to beta=6.4 and the lattice volumes range from 16x16x16x48 to
32x32x32x64.Comment: LaTeX. Self-unpacking, uuencoded tar-compressed fil
Gauge Theories on a 2+2 Anisotropic Lattice
The implementation of gauge theories on a four-dimensional anisotropic
lattice with two distinct lattice spacings is discussed, with special attention
to the case where two axes are finely and two axes are coarsely discretized.
Feynman rules for the Wilson gauge action are derived and the renormalizability
of the theory and the recovery of the continuum limit are analyzed. The
calculation of the gluon propagator and the restoration of Lorentz invariance
in on-shell states is presented to one-loop order in lattice perturbation
theory for on both 2+2 and 3+1 lattices.Comment: 27 pages, uses feynmf. Font compatibility adjuste
Universal Behaviour of the Superfluid Fraction and Tc of He-3 in 99.5% Open Aerogel
We have investigated the superfluid transition of He-3 in a 99.5% porosity
silica aerogel. This very dilute sample shows behaviour intermediary between
bulk He-3 and He-3 confined to the denser aerogels previously studied. We
present data on both the superfluid transition temperature and the superfluid
density and compare our results with previous measurements. Finally, we show
that the suppression of the superfluid transition temperature and suppression
of the superfluid density of He-3 in aerogel follow a universal relation for a
range of aerogel samples.Comment: 4 pages, 5 figures; 1 new figure, minor change
Instantons and Chiral Symmetry on the Lattice
I address the question of how much of QCD in the chiral limit is reproduced
by instantons. After reconstructing the instanton content of smoothed Monte
Carlo lattice configurations, I compare hadron spectroscopy on this instanton
ensemble to the spectroscopy on the original ``physical'' smoothed
configurations using a chirally optimised clover fermion action. By studying
the zero mode zone in simple instances I find that the optimised action gives a
satisfactory description of it. Through the Banks-Casher formula, instantons by
themselves are shown to break chiral symmetry but hadron correlators on the
instanton backgrounds are strongly influenced by free quark propagation. This
results in unnaturally light hadrons and a small splitting between the vector
and the pseudoscalar meson channels. Superimposing a perturbative ensemble of
zero momentum gauge field fluctuations (torons) on the instantons is found to
be enough to eliminate the free quarks and restore the physical hadron
correlators. I argue that the torons that are present only in finite volumes,
are probably needed to compensate the unnaturally large finite size effects due
to the lack of confinement in the instanton ensemble.Comment: 32 pages, LaTeX with 14 eps figure
Confinement and Chiral Symmetry
We illustrate why color deconfines when chiral symmetry is restored in gauge
theories with quarks in the fundamental representation, and while these
transitions do not need to coincide when quarks are in the adjoint
representation, entanglement between them is still present.Comment: 4 pages, 1 figure, proceedings of Quark Matter 200
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