Robert Harlander

Institut für Theoretische Physik und Kosmologie
Fakultät für Mathematik, Informatik, Naturwissenschaften
RWTH Aachen University
52056 Aachen, Germany
fax: +49-241-80-22187
phone: +49-241-80-27045


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Higgs WG

LHC and Philosophy

Robert Harlander: Research Interests -/- Home -/- TTP Karlsruhe -/- CERN Theory -/- HET Brookhaven

Research interests:

Higher order SUSY Higgs production at LHC Electro-weak observables Asymptotic Expansions Top pairs near threshold

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 electron-positron, 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 off-shell top quark and extrapolated to the on-shell 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 electro-weak radiative corrections to top quark pair production above threshold

For top quark pair production at a future linear electron-positron collider, the electro-weak 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/electro-weak contributions are covered is given in [5]


[1] R. Harlander, T. Seidensticker, M. Steinhauser
Corrections of O(alpha alpha-s) to the decay of the Z boson into bottom quarks
Phys. Lett. B 426 (1998) 125 [hep-ph/9712228]

[2] R. Harlander
Higher order corrections to Z decay
Acta Phys. Pol. B 29 (1998) 2691 [hep-ph/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 [hep-ph/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 High-Energy Physics (EPS-HEP 99), Tampere, Finland, 15-21 Jul 1999 [hep-ph/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 [hep-ph/9912262]

Robert Harlander: Research Interests -/- Home -/- TTP Karlsruhe -/- CERN Theory -/- HET Brookhaven
last modified: Jan 5, 2004, by RH