340 research outputs found
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
Increased BOLD signal in the fusiform gyrus during implicit emotion processing in anorexia nervosa
Background
The behavioural literature in anorexia nervosa (AN) has suggested impairments in psychosocial functioning and studies using facial expression processing tasks (FEPT) have reported poorer recognition and slower identification of emotions.
Methods
Functional magnetic resonance imaging (fMRI) was used alongside a FEPT, depicting neutral, mildly happy and happy faces, to examine the neural correlates of implicit emotion processing in AN. Participants were instructed to specify the gender of the faces. Levels of depression, anxiety, obsessive–compulsive symptoms and eating disorder behaviour were obtained and principal component analysis (PCA) was performed to acquire uncorrelated variables.
Results
fMRI analysis revealed a greater blood-oxygenation level dependent (BOLD) response in AN in the right fusiform gyrus to all facial expressions. This response showed a linear increase with the happiness of the facial expression and was found to be stronger in those not taking medication. PCA analysis revealed a single component indicating a greater level of general clinical symptoms.
Conclusion
Neuroimaging findings would suggest that alterations in implicit emotion processing in AN occur during early perceptual processing of social signals and illustrate greater engagement on the FEPT. The lack of separate components using PCA suggests that the questionnaires used might not be suited as predictive measures
The order and corrections to the decay width of the neutral Higgs boson to the pair
We present the analytical expressions for the contributions of the order
and corrections to the
decay width of the Standard Model Higgs boson into the -pair. The
numerical value of the mixed QED and QCD correction of order
is comparable with the previously calculated
terms in the perturbative series for .Comment: LaTeX 5 pages, accepted for publication in Pisma Zh. Eksp. Teor. Fiz.
v 66, N5 (1997
Commensurate Scale Relations in Quantum Chromodynamics
We use the BLM method to show that perturbatively-calculable observables in
QCD can be related to each other without renormalization scale or scheme
ambiguity. We define and study the commensurate scale relations. We show that
the commensurate scales satisfy the renormalization group transitivity rule
which ensures that predictions in PQCD are independent of the choice of an
intermediate renormalization scheme. We generalize the BLM procedure to higher
order. The application of this procedure to relate known physical observables
in QCD gives surprisingly simple results. In particular, the annihilation ratio
and the Bjorken sum rule for polarized electroproduction are
related through simple coefficients, which reinforces the idea of a hidden
symmetry between these two observables.Comment: 35 pages (RevTeX), one PostScript figure included at the end.
SLAC-PUB-6481, UMD Preprint #94-13
Non-perturbative renormalization of lattice operators in coordinate space
We present the first numerical implementation of a non-perturbative
renormalization method for lattice operators, based on the study of correlation
functions in coordinate space at short Euclidean distance. The method is
applied to compute the renormalization constants of bilinear quark operators
for the non-perturbative O(a)-improved Wilson action in the quenched
approximation. The matching with perturbative schemes, such as MS-bar, is
computed at the next-to-leading order in continuum perturbation theory. A
feasibility study of this technique with Neuberger fermions is also presented.Comment: 11 pages and 9 figures, LaTeX2
Effects of technicolor on standard model running couplings
We discuss the running couplings in the standard model, SU(3SU(2U(1, when the Higgs sector is replaced by SU(
technicolor. Particular attention is given to the running of the couplings at
momentum scales where technicolor is nonperturbative, and in this region we
apply a relativistic constituent technifermion model. This model has been
tested against the known running of the QED coupling due to nonperturbative
QCD. An understanding of this low momentum running allows the calculation of
the couplings at a higher scale, , where technicolor becomes
perturbative. We provide numerical values for the changes in the three standard
model couplings between and due to technicolor, assuming
separately ``one doublet'' and ``one family'' technicolor models. The
distinction between a running and walking technicolor coupling is also
considered.Comment: 14 pages of LaTeX, UTPT-94-
Tau Polarimetry with Inclusive Decays
The spin asymmetry parameter characterizing the angular distribution
of the total hadron momentum in the decay of a polarized tau can be calculated
rigorously using perturbative QCD and the operator product expansion.
Perturbative QCD corrections to the free quark result can be
expressed as a power series in and nonperturbative QCD
corrections can be expanded systematically in powers of . The QCD
prediction is . In the decay of a high energy tau into
hadrons, the value of the hadronic energy distribution evaluated
at the maximum hadronic energy fraction can also be calculated
rigorously from QCD.Comment: LateX, 11 pages, no figures, NUHEP-TH-93-
Renormalization-Scale-Invariant PQCD Predictions for R_e+e- and the Bjorken Sum Rule at Next-to-Leading Order
We discuss application of the physical QCD effective charge ,
defined via the heavy-quark potential, in perturbative calculations at
next-to-leading order. When coupled with the Brodsky-Lepage-Mackenzie
prescription for fixing the renormalization scales, the resulting series are
automatically and naturally scale and scheme independent, and represent
unambiguous predictions of perturbative QCD. We consider in detail such
commensurate scale relations for the annihilation ratio
and the Bjorken sum rule. In both cases the improved predictions are in
excellent agreement with experiment.Comment: 13 Latex pages with 5 figures; to be published in Physical Review
Lattice study of vacuum polarization function and determination of strong coupling constant
We calculate the vacuum polarization functions on the lattice using the
overlap fermion formulation.By matching the lattice data at large momentum
scales with the perturbative expansion supplemented by Operator Product
Expansion (OPE), we extract the strong coupling constant in
two-flavor QCD as =
GeV, where the errors are statistical and systematic, respectively. In
addition, from the analysis of the difference between the vector and
axial-vector channels, we obtain some of the four-quark condensates.Comment: 24 pages, 9 figures, enlarged version published in Phys. Rev.
Higgs Decay to Top Quarks at O(\alpha_s^2)
Three-loop corrections to the scalar and pseudo-scalar current correlator are
calculated. By applying the large momentum expansion mass terms up to order
(m^2/q^2)^4 are evaluated analytically. As an application O(\alpha_s^2)
corrections to the decay of a scalar and pseudo-scalar Higgs boson into top
quarks are considered. It is shown that for a Higgs mass not far above the
threshold these higher order mass corrections are necessary to get
reliable results.Comment: Latex, 20 pages, 14 ps-figures. The complete paper, including
figures, is also available via anonymous ftp at
ftp://ttpux2.physik.uni-karlsruhe.de/ , or via www at
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
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