995 research outputs found
Renormalization constants of local operators for Wilson type improved fermions
Perturbative and non-perturbative results are presented on the
renormalization constants of the quark field and the vector, axial-vector,
pseudoscalar, scalar and tensor currents. The perturbative computation, carried
out at one-loop level and up to second order in the lattice spacing, is
performed for a fermion action, which includes the clover term and the twisted
mass parameter yielding results that are applicable for unimproved Wilson
fermions, as well as for improved clover and twisted mass fermions. We consider
ten variants of the Symanzik improved gauge action corresponding to ten
different values of the plaquette coefficients. Non-perturbative results are
obtained using the twisted mass Wilson fermion formulation employing two
degenerate dynamical quarks and the tree-level Symanzik improved gluon action.
The simulations are performed for pion masses in the range of 480 MeV to 260
MeV and at three values of the lattice spacing, a, corresponding to beta=3.9,
4.05, 4.20. For each renormalization factor computed non-perturbatively we
subtract its perturbative O(a^2) terms so that we eliminate part of the cut-off
artifacts. The renormalization constants are converted to MS-bar at a scale of
mu=2 GeV. The perturbative results depend on a large number of parameters and
are made easily accessible to the reader by including them in the distribution
package of this paper, as a Mathematica input file.Comment: 36 pages, 11 figures and 6 tables. The results are included in
electronic form (Mathematica files
Center vortex model for the infrared sector of SU(4) Yang-Mills theory: String tensions and deconfinement transition
A random vortex world-surface model for the infrared sector of SU(4)
Yang-Mills theory is constructed, focusing on the confinement properties and
the behavior at the deconfinement phase transition. Although the corresponding
data from lattice Yang-Mills theory can be reproduced, the model requires a
more complex action and considerably more tuning than the SU(2) and SU(3) cases
studied previously. Its predictive capabilities are accordingly reduced. This
behavior has a definite physical origin, which is elucidated in detail in the
present work. As the number of colors is raised in Yang-Mills theory, the
corresponding infrared effective vortex description cannot indefinitely
continue to rely on dynamics determined purely by vortex world-surface
characteristics; additional color structures present on the vortices begin to
play a role. As evidenced by the modeling effort reported here, definite
signatures of this behavior appear in the case of four colors.Comment: 24 pages, 7 figures containing 8 ps file
A lattice study of the pentaquark states
We present a study of the pentaquark system in quenched lattice QCD using
diquark-diquark and kaon-nucleon local and smeared interpolating fields. We
examine the volume dependence of the spectral weights of local correlators on
lattices of size , and at
. We find that a reliable evaluation of the volume dependence of the
spectral weights requires accurate determination of the correlators at large
time separations. Our main result from the spectral weight analysis in the
pentaquark system is that within our variational basis and statistics we can
not exclude a pentaquark resonance. However our data also do not allow a clear
identification of a pentaquark state since only the spectral weights of the
lowest state can be determined to sufficient accuracy to test for volume
dependence. In the negative parity channel the mass extracted for this state is
very close to the KN threshold whereas in the positive parity channel is about
60% above.Comment: Manuscript expanded, discussion of two-pion system included, a
comment regarding Ref.13 was corrected, version to appear in Phys. Rev. D, 19
figure
On the center-vortex baryonic area law
We correct an unfortunate error in an earlier work of the author, and show
that in center-vortex QCD (gauge group SU(3)) the baryonic area law is the
so-called law, described by a minimal area with three surfaces spanning the
three quark world lines and meeting at a central Steiner line joining the two
common meeting points of the world lines. (The earlier claim was that this area
law was a so-called law, involving three extremal areas spanning the
three pairs of quark world lines.) We give a preliminary discussion of the
extension of these results to . These results are based on the
(correct) baryonic Stokes' theorem given in the earlier work claiming a
law. The -form area law for SU(3) is in agreement with the most
recent lattice calculations.Comment: 5 pages, RevTeX4, 5 .eps figure
The relevance of center vortices
We show remnants of chiral symmetry breaking in the center-projected theory.
We construct and study an unambiguous definition of center vortices.Comment: LATTICE99(confine), 3 pages, 3 figure
The Matter and the Pseudoscalar Densities in Lattice QCD
The matter and the pseudoscalar densities inside a hadron are calculated via
gauge-invariant equal-time correlation functions. A comparison is made between
the charge charge and the matter density distributions for the pion, the rho,
the nucleon and the within the quenched theory, and with two
flavours of dynamical quarks.Comment: Typos corrected; 13 pages, 16 figure
Delta-baryon electromagnetic form factors in lattice QCD
We develop techniques to calculate the four Delta electromagnetic form
factors using lattice QCD, with particular emphasis on the sub-dominant
electric quadrupole form factor that probes deformation of the Delta. Results
are presented for pion masses down to approximately 350 MeV for three cases:
quenched QCD, two flavors of dynamical Wilson quarks, and three flavors of
quarks described by a mixed action combining domain wall valence quarks and
dynamical staggered sea quarks. The magnetic moment of the Delta is chirally
extrapolated to the physical point and the Delta charge density distributions
are discussed.Comment: 4 pages, 5 figure
Field emission properties of nano-composite carbon nitride films
A modified cathodic arc technique has been used to deposit carbon nitride
thin films directly on n+ Si substrates. Transmission Electron Microscopy
showed that clusters of fullerene-like nanoparticles are embedded in the
deposited material. Field emission in vacuum from as-grown films starts at an
electric field strength of 3.8 V/micron. When the films were etched in an
HF:NH4F solution for ten minutes, the threshold field decreased to 2.6
V/micron. The role of the carbon nanoparticles in the field emission process
and the influence of the chemical etching treatment are discussed.Comment: 22 pages, 8 figures, submitted to J. Vac. Sc. Techn.
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