188 research outputs found
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
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
Constraints on the form factors for K --> pi l nu and implications for V_us
Rigorous bounds are established for the expansion coefficients governing the
shape of semileptonic K-->pi form factors. The constraints enforced by
experimental data from tau-->K pi nu eliminate uncertainties associated with
model parameterizations in the determination of |V_us|. The results support the
validity of a powerful expansion that can be applied to other semileptonic
transitions.Comment: 5 pages, 3 figures; references added, version to appear in Phys. Rev.
D alongside hep-ex/060805
Virtual Supersymmetric Corrections in e^+e^- Annihilation
Depending on their masses, Supersymmetric particles can affect various
measurements in Z decay. Among these are the total width (or consequent
extracted value of ), enhancement or suppression of various flavors,
and left-right and forward-backward asymmetries. The latter depend on squark
mass splittings and are, therefore, a possible test of the Supergravity related
predictions. We calculate leading order corrections for these quantities
considering in particular the case of light photino and gluino where the SUSY
effects are enhanced. In this limit the effect on is appreciable,
the effect on is small, and the effect on the asymmetries is extremely
small.Comment: 11 pages, LaTeX, 3 figures, revised, a reference adde
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-
Gluino Contribution to the 3-loop QCD beta function in the Minimal Supersymmetric Standard Model
We deduce the gluino contribution to the three-loop QCD \beta function within
the minimal supersymmetric Standard Model (MSSM) from its standard QCD
expression. The result is a first step in the computation of the full MSSM
three-loop \beta function. In addition, in the case of a light gluino it
provides the strong three-loop SUSY correction to the extrapolation of the
strong coupling constant from the low energy regime to the Z region and up to
the squark threshold.Comment: 11 pages, RevTex, 4 Postscript figur
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
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
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
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