1,536 research outputs found
Fixing the conformal window in QCD
A physical characterization of Landau singularities is emphasized, which
should trace the lower boundary N_f^* of the conformal window in QCD and
supersymmetric QCD. A natural way to disentangle ``perturbative'' from
``non-perturbative'' contributions to amplitudes below N_f^* is suggested.
Assuming an infrared fixed point persists in the perturbative part of the QCD
coupling even below N_f^* leads to the condition \gamma(N_f^*)=1, where \gamma
is the critical exponent. Using the Banks-Zaks expansion, one gets 4<N_f^*<6.
This result is incompatible with the existence of an analogue of Seiberg
duality in QCD. The presence of a negative ultraviolet fixed point is required
both in QCD and in supersymmetric QCD to preserve causality within the
conformal window. Evidence for the existence of such a fixed point in QCD is
provided.Comment: 10 pages, 1 figure, extended version of a talk given at the
QCDNET2000 meeting, Paris, September 11-14 2000; main new material added is
evidence for negative ultraviolet fixed point in QC
Disentangling running coupling and conformal effects in QCD
We investigate the relation between a postulated skeleton expansion and the
conformal limit of QCD. We begin by developing some consequences of an
Abelian-like skeleton expansion, which allows one to disentangle
running-coupling effects from the remaining skeleton coefficients. The latter
are by construction renormalon-free, and hence hopefully better behaved. We
consider a simple ansatz for the expansion, where an observable is written as a
sum of integrals over the running-coupling. We show that in this framework one
can set a unique Brodsky-Lepage-Mackenzie (BLM) scale-setting procedure as an
approximation to the running-coupling integrals, where the BLM coefficients
coincide with the skeleton ones. Alternatively, the running-coupling integrals
can be approximated using the effective charge method. We discuss the
limitations in disentangling running coupling effects in the absence of a
diagrammatic construction of the skeleton expansion. Independently of the
assumed skeleton structure we show that BLM coefficients coincide with the
conformal coefficients defined in the small (Banks-Zaks) limit where
a perturbative infrared fixed-point is present. This interpretation of the BLM
coefficients should explain their previously observed simplicity and smallness.
Numerical examples are critically discussed.Comment: 38 pages; Revised version (to appear in PRD); includes modifications
of section 3 and an added section 4. Sections 6 and 7 of the original version
have been extracted and will be published separatel
Initial design and evaluation of automatic restructurable flight control system concepts
Results of efforts to develop automatic control design procedures for restructurable aircraft control systems is presented. The restructurable aircraft control problem involves designing a fault tolerance control system which can accommodate a wide variety of unanticipated aircraft failure. Under NASA sponsorship, many of the technologies which make such a system possible were developed and tested. Future work will focus on developing a methodology for integrating these technologies and demonstration of a complete system
On the macroion virial contribution to the osmotic pressure in charge-stabilized colloidal suspensions
Our interest goes to the different virial contributions to the equation of
state of charged colloidal suspensions. Neglect of surface effects in the
computation of the colloidal virial term leads to spurious and paradoxical
results. This pitfall is one of the several facets of the danger of a naive
implementation of the so called One Component Model, where the micro-ionic
degrees of freedom are integrated out to only keep in the description the
mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation
of wall induced forces dissolves the paradox brought forth in the naive
approach, provides a consistent description, and confirms that for salt-free
systems, the colloidal contribution to the pressure is dominated by the
micro-ionic one. Much emphasis is put on the no salt case but the situation
with added electrolyte is also discussed
Fixing the renormalisation scheme in NNLO perturbative QCD using conformal limit arguments
We discuss how the renormalisation scheme ambiguities in QCD can be fixed,
when two observables are related, by requiring the coefficients in the
perturbative expansion relating the two observables to have their conformal
limit values, i.e. to be independent of the -function of the
renormalised coupling. We show how the next-to-leading order BLM automatic
scale fixing method can be extended to next-to-next-to-leading order to fix
both the renormalisation scale and in a unique way. As an example we
apply the method to the relation between Bjorken's sum rule and and
compare with experimental data as well as other scheme fixing methods.Comment: 14 pages LaTeX, uses revtex.sty, 1 encapsulated PostScript figur
Infrared renormalons and analyticity structure in pQCD
Relation between the infrared renormalons, the Borel resummation
prescriptions, and the analyticity structure of Green functions in perturbative
QCD (pQCD) is investigated. A specific recently suggested Borel resummation
prescription resulted in the Principal Value and an additional power-suppressed
correction that is consistent with the Operator Product Expansion. Arguments
requiring the finiteness of the result for any power coefficient of the leading
infrared renormalon, and the consistency in the case of the absence of that
renormalon, require that this prescription be modified. The apparently most
natural modification leads to the result represented by the Principal Value.
The analytic structure of the amplitude in the complex coupling plane, obtained
in this way, is consistent with that obtained in the literature by other
methods.Comment: 6 pages, revtex4, 1 eps-figure; improved version - the paragraph
containing Eqs.(18) and (19) is new, as well as the next paragraph; the Title
modified; some references added; version to appear in Phys. Rev.
Relating Physical Observables in QCD without Scale-Scheme Ambiguity
We discuss the St\"uckelberg-Peterman extended renormalization group
equations in perturbative QCD, which express the invariance of physical
observables under renormalization-scale and scheme-parameter transformations.
We introduce a universal coupling function that covers all possible choices of
scale and scheme. Any perturbative series in QCD is shown to be equivalent to a
particular point in this function. This function can be computed from a set of
first-order differential equations involving the extended beta functions. We
propose the use of these evolution equations instead of perturbative series for
numerical evaluation of physical observables. This formalism is free of
scale-scheme ambiguity and allows a reliable error analysis of higher-order
corrections. It also provides a precise definition for as the pole in the associated 't Hooft scheme. A concrete application to
is presented.Comment: Plain TEX, 4 figures (available upon request), 22 pages,
DOE/ER/40322-17
Why Pad\'e Approximants reduce the Renormalization-Scale dependence in QFT?
We prove that in the limit where the beta function is dominated by the 1-loop
contribution (``large beta_0 limit'') diagonal Pad\'e Approximants (PA's) of
perturbative series become exactly renormalization scale (RS) independent. This
symmetry suggest that diagonal PA's are resumming correctly contributions from
higher order diagrams that are responsible for the renormalization of the
coupling-constant. Non-diagonal PA's are not exactly invariant, but generally
reduce the RS dependence as compared to partial-sums. In physical cases,
higher-order corrections in the beta function break the symmetry softly,
introducing a small scale and scheme dependence. We also compare the Pad\'e
resummation with the BLM method. We find that in the large-N_f limit using the
BLM scale is identical to resumming the series by a non-diagonal PA.Comment: 25 pages, LateX. Replaced so that the figures would fit into the page
siz
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