662 research outputs found
O(\alpha_s^2) Corrections to Top Quark Production at Colliders
In this article we evaluate mass corrections up to to the
three-loop polarization function induced by an axial-vector current. Special
emphasis is put on the evaluation of the singlet diagram which is absent in the
vector case. As a physical application corrections to the
production of top quarks at future colliders is considered. It is
demonstrated that for center of mass energies GeV the
inclusion of the first seven terms into the cross section leads to a reliable
description.Comment: LaTeX, 13 pages, 10 figures included as ps-files. 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
Towards Higgs boson production in gluon fusion to NNLO in the MSSM
We consider the Higgs boson production in the gluon-fusion channel to
next-to-next-to-leading order within the Minimal Supersymmetric Standard Model.
In particular, we present analytical results for the matching coefficient of
the effective theory and study its influence on the total production cross
section in the limit where the masses of all MSSM particles coincide. For
supersymmetric masses below 500 GeV it is possible to find parameters leading
to a significant enhancement of the Standard Model cross section, the
-factors, however, change only marginally.Comment: 20 pages; v2: modification of discussion of numerical effect, version
to appear in EPJC; v3: eq.(18) corrected, minor correction
Complete Corrections of O(\alpha\alpha_s) to the Decay of the Z Boson into Bottom Quarks
For the vertex corrections to the partial decay rate
involving the top quark only the leading terms of order in the
expansion are known. In this work we compute the missing
next-to-leading corrections. Thus at the complete
corrections to the decay of the Z boson into bottom quarks are at hand.Comment: Latex, 11 pages, 1 figure included as ps-file. Two references
changed. The complete paper is also available via anonymous ftp at
ftp://ttpux2.physik.uni-karlsruhe.de/ttp97/ttp97-52/ or via www at
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
The four-loop DRED gauge beta-function and fermion mass anomalous dimension for general gauge groups
We present four-loop results for the gauge beta-function and the fermion mass
anomalous dimension for a gauge theory with a general gauge group and a
multiplet of fermions transforming according to an arbitrary representation,
calculated using the dimensional reduction scheme. In the special case of a
supersymmetric theory we confirm previous calculations of both the gauge
beta-function and the gaugino mass beta-function.Comment: 44 pages, added references (v2) minor changes (v3
On the NLO QCD corrections to the production of the heaviest neutral Higgs scalar in the MSSM
We present a calculation of the two-loop top-stop-gluino contributions to
Higgs production via gluon fusion in the MSSM. By means of an asymptotic
expansion in the heavy particle masses, we obtain explicit and compact analytic
formulae that are valid when the Higgs and the top quark are lighter than stops
and gluino, without assuming a specific hierarchy between the Higgs mass and
the top mass. Being applicable to the heaviest Higgs scalar in a significant
region of the MSSM parameter space, our results complement earlier ones
obtained with a Taylor expansion in the Higgs mass, and can be easily
implemented in computer codes to provide an efficient and accurate
determination of the Higgs production cross section.Comment: 18 pages, 4 figure
Running mass of the b-quark in QCD and SUSY QCD
The running mass of the b-quark defined in DRbar-scheme is one of the
important parameters of SUSY QCD. To find its value it should be related to
some known experimental input. In this paper the b-quark running mass defined
in nonsupersymmetric QCD is chosen for determination of corresponding parameter
in SUSY QCD. The relation between these two quantities is found by considering
five-flavor QCD as an effective theory obtained from its supersymmetric
extension. A numerical analysis of the calculated two-loop relation and its
impact on the MSSM spectrum is discussed. Since for nonsupersymmetric models
MSbar-scheme is more natural than DRbar, we also propose a new procedure that
allows one to calculate relations between MSbar- and DRbar-parameters.
Unphysical epsilon-scalars that give rise to the difference between mentioned
schemes are assumed to be heavy and decoupled in the same way as physical
degrees of freedom. By means of this method it is possible to ``catch two
rabbits'', i.e., decouple heavy particles and turn from DRbar to MSbar, at the
same time. Explicit two-loop example of DRbar -> MSbar transition is given in
the context of QCD. The advantages and disadvantages of the method are briefly
discussed.Comment: 33 pages, 6 figures, 1 table, typos corrected, added references
Two-loop matching coefficients for the strong coupling in the MSSM
When relating the strong coupling , measured at the scale of the
boson mass, to its numerical value at some higher energy, for example the
scale of Grand Unification, it is important to include higher order corrections
both in the running of and the decoupling of the heavy particles. We
compute the two-loop matching coefficients for within the Minimal
Supersymmetric Standard Model (MSSM) which are necessary for a consistent
three-loop evolution of the strong coupling constant. Different scenarios for
the hierarchy of the supersymmetric scales are considered and the numerical
effects are discussed. We find that the three-loop effects can be as large as
and sometimes even larger than the uncertainty induced by the current
experimental accuracy of .Comment: 22 pages, 8 figures (13 ps/eps-files
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