489 research outputs found
Complete Renormalization Group Improvement-Avoiding Factorization and Renormalization Scale Dependence in QCD Predictions
For moments of leptoproduction structure functions we show that all
dependence on the renormalization and factorization scales disappears, provided
that all the ultraviolet logarithms involving the physical energy scale Q are
completely resummed. The approach is closely related to Grunberg's method of
Effective Charges. A direct and simple method for extracting the universal
dimensional transmutation parameter of QCD from experimental data is advocated.Comment: 16 pages, no figure
Modelling the Longitudinal Properties of Financial Ratios of European Firms
The use of financial ratios by analysts to compare the performance of firms from one accounting period to the next is of growing importance with continued European economic integration. Recent studies suggest that the individual component series of financial ratios exhibit nonstationarity which is not eliminated by the ratio transformation. In this paper, w e derive a generalised model that incorporates stochastic and deterministic trends and allows for restricted and unrestricted proportionate growth in the ratio numerator and denominator. When the individual firm series are included in a panel structure with large N and small T, we are unable to reject convincingly a joint hypothesis of nonstationarity, whilst in about one third of the individual firm panels there is no evidence of a unit root. Although the components of financial ratios are correlated variables, our estimates show that any cointegrating effects decay rapidly.financial ratios, nonstationarity, proportionate growth, cointegration, panel methods
Renormalization-Scheme-Independent Perturbation Theory by Resumming Logarithms
Results of perturbation theory in quantum field theory generally depend on
the renormalization scheme that is in use. In particular, they depend on the
scale. We try to make perturbation theory scheme invariant by re-expanding with
respect to a scheme invariant quantity. Furthermore, we investigate whether the
potentially large logarithms in such an expansion cause inaccuracy and how this
can be improved.Comment: 15 pages, 2 figures. First replaced version: added one reference.
Second replaced version: added two references, mention effective-charge
method, added remarks about further research possibilities, changed size of
graphs a littl
Direct Extraction of QCD Lambda MS-bar from e+e- Jet Observables
We directly fit the QCD dimensional transmutation parameter, Lambda MS-bar,
to experimental data on e+e- jet observables, making use of next-to-leading
order (NLO) perturbative calculations. In this procedure there is no need to
mention, let alone to arbitrarily vary, the unphysical renormalisation scale
mu, and one avoids the spurious and meaningless ``theoretical error''
associated with standard alpha_s determinations. PETRA, SLD, and LEP data are
considered in the analysis. An attempt is made to estimate the importance of
uncalculated next-NLO and higher order perturbative corrections, and power
corrections, by studying the scatter in the values of Lambda MS-bar obtained
for different observables.Comment: 46 pages, 22 figure
A Crystal Structure of the Bifunctional Antibiotic Simocyclinone D8, Bound to DNA Gyrase
Simocyclinones are bifunctional antibiotics that inhibit bacterial DNA gyrase by preventing DNA binding to the enzyme. We report the crystal structure of the complex formed between the N-terminal domain of the Escherichia coli gyrase A subunit and simocyclinone D8, revealing two binding pockets that separately accommodate the aminocoumarin and polyketide moieties of the antibiotic. These are close to, but distinct from, the quinolone-binding site, consistent with our observations that several mutations in this region confer resistance to both agents. Biochemical studies show that the individual moieties of simocyclinone D8 are comparatively weak inhibitors of gyrase relative to the parent compound, but their combination generates a more potent inhibitor. Our results should facilitate the design of drug molecules that target these unexploited binding pockets
RS-invariant all-orders renormalon resummations for some QCD observables
We propose a renormalon-inspired resummation of QCD perturbation theory based
on approximating the renormalization scheme (RS) invariant effective charge
beta-function coefficients by the portion containing the highest power of
=--, for SU() QCD with quark flavours.
This can be accomplished using exact large- all-orders results. The
resulting resummation is RS-invariant and the exact next-to-leading order (NLO)
and next-to-NLO (NNLO) coefficients in any RS are included. This improves on a
previously employed naive resummation of the leading- piece of the
perturbative coefficients which is RS-dependent, making its comparison with
fixed-order perturbative results ambiguous. The RS-invariant resummation is
used to assess the reliability of fixed-order perturbation theory for the
-ratio, the analogous -lepton decay ratio , and
Deep Inelastic Scattering (DIS) sum rules, by comparing it with the exact NNLO
results in the effective charge RS. For the -ratio and , where
large-order perturbative behaviour is dominated by a leading ultra-violet
renormalon singularity, the comparison indicates fixed-order perturbation
theory to be very reliable. For DIS sum rules, which have a leading infra-red
renormalon singularity, the performance is rather poor. In this way we estimate
that at LEP/SLD energies ideal data on the -ratio could determine
to three-significant figures, and for the we
estimate a theoretical uncertainty
corresponding to . This encouragingly small
uncertainty is much less than has recently been deduced from comparison with
the ambiguous naive resummation.Comment: 25 pages, uses LaTeX, 12 Postscript figures, epsfig.sty 'elsart.sty'
and 'elsart12.sty' are available via anonymous-ftp at
ftp://ftp.tex.ac.uk/tex-archive/macros/latex/contrib/supported/elsevie
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
Renormalon-inspired resummations for vector and scalar correlators- estimating the uncertainty in {alpha}_{s}({m}_{\tau}^{2}) and and {\alpha}({M}_{Z}^{2})
We perform an all-orders resummation of the QCD Adler D-function for the
vector correlator, in which the portion of perturbative coefficients involving
the leading power of b, the first beta-function coefficient, is resummed. To
avoid a renormalization scale dependence when we match the resummation to the
exactly known NLO and NNLO results, we employ the Complete Renormalization
Group Improvement (CORGI) approach. These fixed-order and resummed CORGI
results are analytically continued by numerically performing a contour integral
to obtain corresponding fixed and all-orders ``contour-improved'' results for
the e+e- R-ratio ands its tau decay analogue R_{\tau}. The difference between
these fixed-order and all-order results is used to estimate the uncertainty in
the extraction of {alpha}_{s}({M}_{Z}^{2}} from R_{\tau} measurements, and that
in the QED coupling {\alpha}({M}_{Z}^{2}) due to hadronic corrections related
to R. Analogous resummations for the scalar correlator are performed, and used
to assess the uncertainty in the Higgs decay width to a heavy quark pair. We
point out that CORGI fixed-order contour-improved results for R and the Higgs
decay width, can be given explicitly in terms of the Lambert-W function and
hypergeometric functions, avoiding the need for numerical integration.Comment: 33 pages, 8 figure
ALEPH Tau Spectral Functions and QCD
Hadronic decays provide a clean laboratory for the precise study of
quantum chromodynamics (QCD). Observables based on the spectral functions of
hadronic decays can be related to QCD quark-level calculations to
determine fundamental quantities like the strong coupling constant, quark and
gluon condensates. Using the ALEPH spectral functions and branching ratios,
complemented by some other available measurements, and a revisited analysis of
the theoretical framework, the value \asm = 0.345 \pm 0.004_{\rm exp} \pm
0.009_{\rm th} is obtained. Taken together with the determination of \asZ from
the global electroweak fit, this result leads to the most accurate test of
asymptotic freedom: the value of the logarithmic slope of is
found to agree with QCD at a precision of 4%. The value of \asZ obtained from
decays is \asZ = 0.1215 \pm 0.0004_{\rm exp} \pm 0.0010_{\rm th} \pm
0.0005_{\rm evol}
= 0.1215 \pm 0.0012.Comment: 14 pages, 5 figures, invited talk at the Tau06 International
Workshop, Pisa, September 19-22 200
Using an Effective Charges Method to extract Lambda-MS-bar from event shape moments in e+e- annihilation
We use an Effective Charges (ECH) method to extract Lambda-MS-bar, and hence
alpha_s(M_z), from event shape moments in e+e- annihilation. We compare these
results with ones obtained using standard MS-bar perturbation theory. The ECH
method at NLO is found to perform better than standard MS-bar perturbation
theory when applied to means of event shape observables. For example, when we
apply the NLO ECH method to we get alpha_s(M_z)=0.1193\pm0.0003. However
ECH at NNLO is found to work less well than ECH at NLO, and the ECH method also
fails to describe data for higher moments of event shapes. We attempt to
explain this by considering the ECH beta-function as an asymptotic series. We
also examine the effect of adding two different models for non-perturbative
power corrections to the perturbative approximation given by the ECH method and
MS-bar perturbation theory. Whilst only small power corrections are required
when using ECH at NLO, it is found that these models are insufficient to
couteract the undesirable behaviour of ECH at NNLO.Comment: 43 pages, 16 figures, Extended Conclusions section, minor textual
editing, slight changes to some tabulated values, overall conclusions
unchange
- …