159 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
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
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-
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
On the QCD perturbative expansion for e^+ e^- --> hadrons
We study the perturbative QCD series for the hadronic width of the Z boson.
We sum a class of large ``pi^2 terms'' and reorganize the series so as to
minimize ``renormalon'' effects. We also consider the renormalization
scheme-scale ambiguity of the perturbative results. We find that, with three
nontrivial known terms in the perturbative expansion, the treatment of the pi^2
terms is quite important, while renormalon effects are less important. The
measured hadronic width of the Z is often used to determine the value of
alpha_s(M_Z^2). A standard method is to use the perturbative expansion for the
width truncated at order alpha_s^3 in the MS-bar scheme with scale mu = M_Z. We
estimate that the determined value of alpha_s(M_Z^2) should be increased by
0.6% compared to the value extracted with this standard method. After this
adjustment for pi^2 and renormalon effects, we estimate that the uncertainty in
alpha_s(M_Z^2) arising from QCD theory is about 0.4%. This is, of course, much
less than the experimental uncertainty of about 5%.Comment: 23 pages, REVTEX3.0, uses epsf.tex to insert figures; with 6 figures
in encapsulated postscript for
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