522 research outputs found
The topological susceptibility in `full' (UK)QCD
We report first calculations of the topological susceptibility measured using
the field theoretic method on SU(3) gauge configurations produced by the UKQCD
collaboration with two flavours of dynamical, improved, Wilson fermions. Using
three ensembles with matched lattice spacing but differing sea quark mass we
find that hybrid Monte Carlo simulation appears to explore the topological
sectors efficiently, and a topological susceptibility consistent with
increasing linearly with the quark mass.Comment: LaTeX. 4 PostScript figures. Contribution to LATTICE99(topology
Instantons and Monopoles in the Maximally Abelian Gauge
We study the Abelian projection of SU(2) instantons in the Maximally Abelian
gauge. We find that in this gauge an isolated instanton produces a closed
monopole loop within its core and the size of this loop increases with the core
size. We show that this result is robust against the introduction of small
quantum fluctuations. We investigate the effects of neighbouring
(anti)instantons upon each other and show how overlapping (anti)instantons can
generate larger monopole loops. We find, however, that in fields that are
typical of the fully quantised vacuum only some of the large monopole loops
that are important for confinement have a topological origin. We comment on
what this may imply for the role of instantons in confinement and chiral
symmetry breaking.Comment: 14 pages LaTeX plus 5 PostScript figures. Uses epsf.sty.
Self-unpacking, uuencoded tar-compressed fil
Z2 monopoles in D=2+1 SU(2) lattice gauge theory
We calculate the Euclidean action of a pair of Z2 monopoles (instantons), as
a function of their spatial separation, in D=2+1 SU(2) lattice gauge theory. We
do so both above and below the deconfining transition at T=Tc. At high T, and
at large separation, we find that the monopole `interaction' grows linearly
with distance: the flux between the monopoles forms a flux tube (exactly like a
finite portion of a Z2 domain wall) so that the monopoles are linearly
confined. At short distances the interaction is well described by a Coulomb
interaction with, at most, a very small screening mass, possibly equal to the
Debye electric screening mass. At low T the interaction can be described by a
simple screened Coulomb (i.e. Yukawa) interaction with a screening mass that
can be interpreted as the mass of a `constituent gluon'. None of this is
unexpected, but it helps to resolve some apparent controversies in the recent
literature.Comment: 14 pages, 2 figure
Vortices and confinement in hot and cold D=2+1 gauge theories
We calculate the variation with temperature of the vortex free energy in
D=2+1 SU(2) lattice gauge theories. We do so both above and below the
deconfining transition at T=Tc. We find that this quantity is zero at all T for
large enough volumes. For T<Tc this observation is consistent with the fact
that the phase is linearly confining; while for T>Tc it is consistent with the
conventional expectation of `spatial' linear confinement. In small spatial
volumes this quantity is shown to be non-zero. The way it decreases to zero
with increasing volume is shown to be controlled by the (spatial) string
tension and it has the functional form one would expect if the vortices being
studied were responsible for the confinement at low T, and for the `spatial'
confinement at large T. We also discuss in detail some of the direct numerical
evidence for a non-zero spatial string tension at high T, and we show that the
observed linearity of the (spatial) potential extends over distances that are
large compared to typical high-T length scales.Comment: 27 pages, 6 figure
Gauge Theory in d=2+1 at High Temperature: Z_N interface
We calculate on the lattice the interface tension in the SU(2) pure gauge
theory in d=2+1 at high temperature. The result is compared to the perturbative
prediction. The agreement confirms applicability of the perturbation theory in
this case.Comment: Lattice'94 contribution, 3pp, uuencoded compressed postscrip
Topology in CP(N-1) models: a critical comparison of different cooling techniques
Various cooling methods, including a recently introduced one which smoothes
out only quantum fluctuations larger than a given threshold, are applied to the
study of topology in 2d CP(N-1) models. A critical comparison of their
properties is performed.Comment: Poster at LATTICE99(Topology and confinement), 3 pages, 5 eps
figures, uses espcrc2.st
The topological susceptibility and f_pi from lattice QCD
We study the topological susceptibility, chi, in QCD with two quark flavours
using lattice field configurations that have been produced with an O(a)
improved quark action. We find clear evidence for the expected suppression at
small quark mass, and examine the variation of chi with this mass. The
resulting estimate of the pion decay constant, f_pi = 105 +/- 6 ^{+18}_{-10}
MeV, is consistent with the experimental value of ~93 MeV. We compare chi to
the large-N_c prediction and find consistency over a large range of quark
masses. We discuss the benefits of the non-perturbative action improvement
scheme and of the stategy of keeping the lattice spacing (nearly) fixed as the
quark mass is varied. We compare our results with other studies and suggest why
such a quark mass dependence has not always been seen.Comment: LaTeX. Supercedes our preprint hep-ph/0004180. Version to appear in
Phys. Lett.
The string tension in the maximally Abelian gauge after smoothing
We apply smoothing to SU(2) lattice field configurations in 3+1 dimensions
before fixing to the maximally Abelian gauge. The Abelian projected string
tension is shown to be stable under this, whilst the monopole string tension
declines by O(30%). Blocking of the SU(2) fields reduces this effect, but the
use of extended monopole definitions does not. We discuss these results in the
context of additional confining excitations in the U(1) vacuum.Comment: LATTICE98(confine
Scalar-isoscalar states in the large-Nc Regge approach
Scalar-isoscalar states (0++) are investigated within the large-Nc Regge
approach. We elaborate on the consequences of including the lightest f0(600)
scalar-isoscalar state into such an analysis, where the position of f0(600)
fits very well into the pattern of the radial Regge trajectory. Furthermore, we
point out that the pion and nucleon spin-0 gravitational form factors, recently
measured on the lattice, provide valuable information on the low-mass spectrum
of the scalar-isoscalar states on the basis of the scalar-meson dominance in
the spin-0 channel. Through the fits to these data we find m_sigma= 450-600
MeV. We compare the predictions of various fits and methods. An analysis of the
QCD condensates in the two-point correlators provides further constraints on
the parameters of the scalar-isoscalar sector. We find that a simple two-state
model suggests a meson nature of f0(600), and a glueball nature of f0(980),
which naturally explains the ratios of various coupling constants. Finally, we
note that the fine-tuned condition of the vanishing dimension-2 condensate in
the Regge approach with infinitely many scalar-isoscalar states yields a
reasonable value for the mass of the lighest glueball state.Comment: 18 pages, 13 figures. Typos corrected. Remarks and references adde
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