157 research outputs found
Analyses of shuttle orbiter approach and landing conditions
A study of one shuttle orbiter approach and landing conditions are summarized. Causes of observed PIO like flight deficiencies are identified and potential cures are examined. Closed loop pilot/vehicle analyses are described and path/attitude stability boundaries defined. The latter novel technique proved of great value in delineating and illustrating the basic causes of this multiloop pilot control problem. The analytical results are shown to be consistent with flight test and fixed base simulation. Conclusions are drawn relating to possible improvements of the shuttle orbiter/digital flight control system
On the screening of the potential between adjoint sources in
We calculate the potential between adjoint sources in pure gauge
theory in three dimensions. We investigate whether the potential saturates at
large separations due to the creation of a pair of gluelumps, colour-singlet
states formed when glue binds to an adjoint source.Comment: 3 pages, uuencoded Z-compressed postscript file, contribution to
Lattice '9
Breakdown of large-N quenched reduction in SU(N) lattice gauge theories
We study the validity of the large-N equivalence between four-dimensional
SU(N) lattice gauge theory and its momentum quenched version--the Quenched
Eguchi-Kawai (QEK) model. We find that the assumptions needed for the proofs of
equivalence do not automatically follow from the quenching prescription. We use
weak-coupling arguments to show that large-N equivalence is in fact likely to
break down in the QEK model, and that this is due to dynamically generated
correlations between different Euclidean components of the gauge fields. We
then use Monte-Carlo simulations at intermediate couplings with 20 <= N <= 200
to provide strong evidence for the presence of these correlations and for the
consequent breakdown of reduction. This evidence includes a large discrepancy
between the transition coupling of the "bulk" transition in lattice gauge
theories and the coupling at which the QEK model goes through a strongly
first-order transition. To accurately measure this discrepancy we adapt the
recently introduced Wang-Landau algorithm to gauge theories.Comment: 51 pages, 16 figures, Published verion. Historical inaccuracies in
the review of the quenched Eguchi-Kawai model are corrected, discussion on
reduction at strong-coupling added, references updated, typos corrected. No
changes to results or conclusion
``GLUELUMP'' SPECTRUM AND ADJOINT SOURCE POTENTIAL IN LATTICE QCD
We calculate the potential between ``quarks'' which are in the adjoint
representation of SU(2) color in the three-dimensional lattice theory. We work
in the scaling region of the theory and at large quark separations . We also
calculate the masses of color-singlet bound states formed by coupling
an adjoint quark to adjoint glue (``gluelumps''). Good scaling behavior is
found for the masses of both magnetic (angular momentum ) and electric
() gluelumps, and the magnetic gluelump is found to be the lowest-lying
state. It is naively expected that the potential for adjoint quarks should
saturate above a separation where it becomes energetically
favorable to produce a pair of gluelumps. We obtain a good estimate of the
naive screening distance . However we find little evidence of
saturation in the potential out to separations of about twice .Comment: 8 pages plus 8 figures in 2 postscript files (uuencoded
Magnetic Z(N) symmetry in 2+1 dimensions
This review describes the role of magnetic symmetry in 2+1 dimensional gauge
theories. In confining theories without matter fields in fundamental
representation the magnetic symmetry is spontaneously broken. Under some mild
assumptions, the low-energy dynamics is determined universally by this
spontaneous breaking phenomenon. The degrees of freedom in the effective theory
are magnetic vortices. Their role in confining dynamics is similar to that
played by pions and sigma in the chiral symmetry breaking dynamics.
I give an explicit derivation of the effective theory in (2+1)-dimensional
weakly coupled confining models and argue that it remains qualitatively the
same in strongly coupled (2+1)-dimensional gluodynamics. Confinement in this
effective theory is a very simple classical statement about the long range
interaction between topological solitons, which follows (as a result of a
simple direct classical calculation) from the structure of the effective
Lagrangian. I show that if fundamentally charged dynamical fields are present
the magnetic symmetry becomes local rather than global. The modifications to
the effective low energy description in the case of heavy dynamical fundamental
matter are discussed. This effective lagrangian naturally yields a bag like
description of baryonic excitations. I also discuss the fate of the magnetic
symmetry in gauge theories with the Chern-Simons term
String breaking by dynamical fermions in three-dimensional lattice QCD
The first observation is made of hadronic string breaking due to dynamical
fermions in zero temperature lattice QCD. The simulations are done for SU(2)
color in three dimensions, with two flavors of staggered fermions. The results
have clear implications for the large scale simulations that are being done to
search (so far, without success) for string breaking in four-dimensional QCD.
In particular, string breaking is readily observed using only Wilson loops to
excite a static quark-antiquark pair. Improved actions on coarse lattices are
used, providing an extremely efficient means to access the quark separations
and propagation times at which string breaking occurs.Comment: Revised version to appear in Physical Review D, has additional
discussion of the results, additional references, modified title, larger
figure
Numerical study of SU(2) Yang-Mills theory with gluinos
We report on a numerical investigation of the SU(2) gauge theory with
gluinos.
The low-lying spectrum in bosonic and fermionic channels is determined.
Improvements of the multi-bosonic algorithm are discussed.Comment: latex, 3 pages, 4 figures; Poster presented by K. Spanderen at
LATTICE9
Confining strings in SU(N) gauge theories
We calculate the string tensions of -strings in SU() gauge theories in
both 3 and 4 dimensions. In D=3+1, we find that the ratio of the string
tension to the fundamental string tension is consistent, at the level, with both the M(-theory)QCD-inspired conjecture and with
`Casimir scaling'. In D=2+1 we see a definite deviation from the MQCD formula,
as well as a much smaller but still significant deviation from Casimir scaling.
We find that in both D=2+1 and D=3+1 the high temperature spatial -string
tensions also satisfy approximate Casimir scaling. We point out that
approximate Casimir scaling arises naturally if the cross-section of the flux
tube is nearly independent of the flux carried, and that this will occur in an
effective dual superconducting description, if we are in the deep-London limit.
We estimate, numerically, the intrinsic width of -strings in D=2+1 and
indeed find little variation with . In addition to the stable -strings we
investigate some ofthe unstable strings, finding in D=2+1 that they satisfy
(approximate) Casimir scaling. We also investigate the basic assumption that
confining flux tubes are described by an effective string theory at large
distances. We estimate the coefficient of the universal L\"uscher correction
from periodic strings that are longer than 1 fermi, and find in
D=3+1 and in D=2+1. These values are within of the
simple bosonic string values and are inconsistent with other simple effective
string theories.Comment: 57 pages, 11 figures. Errors on fits reduced by altering the analysis
to a standard one. Conclusions unchanged; note addedchanged. Some typos
correcte
Effective matrix model for deconfinement in pure gauge theories
We construct matrix models for the deconfining phase transition in SU(N)
gauge theories, without dynamical quarks, at a nonzero temperature T. We
generalize models with zero and one free parameter to study a model with two
free parameters: besides perturbative terms ~T^4, we introduce terms ~T^2 and
~T^0. The two N-dependent parameters are determined by fitting to data from
numerical simulations on the lattice for the pressure, including the latent
heat. Good agreement is found for the pressure in the semi-quark gluon plasma
(QGP), which is the region from Tc, the critical temperature, to about ~4 Tc.
Above ~1.2 Tc, the pressure is a sum of a perturbative term, ~ +T^4, and a
simple non-perturbative term, essentially just a constant times ~ -Tc^2 T^2.
For the pressure, the details of the matrix model only enter within a very
narrow window, from Tc to ~1.2 Tc, whose width does not change significantly
with N. Without further adjustment, the model also agrees well with lattice
data for the 't Hooft loop. This is notable, because in contrast to the
pressure, the 't Hooft loop is sensitive to the details of the matrix model
over the entire semi-QGP. For the (renormalized) Polyakov loop, though, our
results disagree sharply with those from the lattice. Matrix models provide a
natural and generic explanation for why the deconfining phase transition in
SU(N) gauge theories is of first order not just for three, but also for four or
more colors. Lastly, we consider gauge theories where there is no strict order
parameter for deconfinement, such as for a G(2) gauge group. To agree with
lattice measurements, in the G(2) matrix model it is essential to add terms
which generate complete eigenvalue repulsion in the confining phase.Comment: 80 pages, 26 figure
On the chirality of quark modes
A model for the QCD vacuum based on a domainlike structured background gluon
field with definite duality attributed to the domains has been shown elsewhere
to give confinement of static quarks, a reasonable value for the topological
susceptibility and indications that chiral symmetry is spontaneously broken. In
this paper we study in detail the eigenvalue problem for the Dirac operator in
such a gluon mean field. A study of the local chirality parameter shows that
the lowest nonzero eigenmodes possess a definite mean chirality correlated with
the duality of a given domain. A probability distribution of the local
chirality qualitatively reproduces histograms seen in lattice simulations.Comment: RevTeX4, 5 figures, 14 page
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