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Robert Harlander:
Research Interests
/ Home
/ TTP Karlsruhe
/ CERN Theory
/ HET
Brookhaven
Electroweak observables in high order perturbation theory
Z boson decay into bottom quarks
The huge amount of data at the ↳Z boson
resonance collected by the four ↳LEP experiments
provides a unique opportunity to test the
↳Standard Model in many
of its details.
This requires the theoretical knowledge of the relevant observables
with high accuracy. However, it is impossible to evaluate sufficiently high
orders of ↳perturbation theory
exactly. For example, the decay rate
for the Z boson into ↳bottom quarks
was previously known only in its
leading term in the limit of a heavy ↳top quark.
It was the concern of [1] to decrease this
theoretical
uncertainty by reproducing the full top mass dependence in this rate. (Some
details about the calculation can also be found in [2]). The result was particularly
helpful for the final analysis of the complete set of Z resonance data (see,
e.g., the report by D. Bardin, M. Grünewald, and G. Passarino, [ref]).
Second order QCD corrections to top decay
The top quark is certainly one of the most promising objects of investigation
at future particle accelerators like the
↳LHC, a linear electronpositron,
or a muon collider. It is necessary to know its expected properties as imposed
by the Standard Model to sufficiently high precision in order to be able
to trace even tiny deviations from them in these experiments.
Diagram whose imaginary part contributes to the top decay rate in second
order QCD.
The expected precision for the top quark width is about the same order
of magnitude as the first order QCD corrections, so it is important to make
sure that the second order corrections are under control. The tools to compute
the corresponding Feynman diagrams fully analytically are known only since
recently (see the works by K. Melnikov and T. van Ritbergen, [ref, ref]). However, asymptotic
expansions allowed an excellent prediction for this quantity by extrapolation
from a certain, unphysical limit in phase space to the actual physical point.
Two rather complementary approaches have been assumed by two different groups.
A. Czarnecki and K. Melnikov [ref] performed an expansion
in the difference of the bottom and top quark mass. The calculation of [3] (see also [4]), on the other hand,
assumed a slightly offshell top quark and extrapolated to the onshell point
by applying Padé approximations (see here
for a review). The excellent agreement of both approaches gives confidence
to the result and demonstrates the strength of expansion techniques in general.
Combination of QCD and electroweak radiative corrections to top quark
pair production above threshold
For top quark pair production at a future linear electronpositron collider,
the electroweak corrections are known to first order, the QCD induced ones
to second (for not too small energies even to third) order in perturbation
theory. A prescription that combines both types of corrections in such a
way that even the leading mixed QCD/electroweak contributions are covered
is given in [5].
Literature:
[1] 
R. Harlander, T. Seidensticker, M. Steinhauser 
 Corrections of O(alpha alphas)
to the decay of the Z boson into bottom quarks 

Phys. Lett. B 426 (1998) 125 [hepph/9712228]




[2] 
R. Harlander 
 Higher order corrections to Z decay 

Acta Phys. Pol. B 29 (1998) 2691 [hepph/9806524]




[3] 
K.G. Chetyrkin, R. Harlander,
T. Seidensticker, M. Steinhauser 
 Second order QCD corrections to Gamma(t→Wb) 

Phys. Rev. D 60 (1999) 114015 [hepph/9906273]




[4] 
K.G. Chetyrkin, R. Harlander,
T. Seidensticker, M. Steinhauser 
 Second order QCD corrections to the top decay rate 

Proc. of the International Europhysics Conference on
HighEnergy Physics (EPSHEP 99), Tampere,
Finland, 1521 Jul 1999 [hepph/9910339]




[5] 
J.H. Kühn, T. Hahn, R. Harlander 
 Top production above threshold:
Electroweak and QCD corrections combined 

Proc. of the 4th International Workshop
on Linear Colliders (LCWS 99),
Sitges, Barcelona, Spain, 28 Apr  5 May 1999 [hepph/9912262]




Robert Harlander: Research Interests
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last modified: Jan 5, 2004, by RH
