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


Brief CV

– Research –







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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

Top quark pair production near threshold

Lippmann-Schwinger equation

Because the lifetime of a top quark is shorter than the typical time scale for a bound toponium state, the top--anti-top system turns out to be a unique testing ground for QCD. The production cross section near threshold can be computed by solving a Lippmann-Schwinger-like equation which involves the QCD potential.

Lippmann-Schwinger equation in diagrammatical form.
The shaded blob denotes the tt-bar vertex in the threshold region.

Polarization of top quarks

Our contribution in this field was to investigate the polarization of the top quarks in the threshold region. One of the main results ([1], [2]) was that the maximal component of the polarization vector which is parallel to the electron beam is hardly affected by threshold effects. It is therefore well suited for experimental studies of the top--anti-top system at a future linear collider.

Rescattering corrections

Rescattering corrections between the top quark and its decay products near threshold are formally of the same order of magnitude as the non-maximal components of the polarization vector. Therefore it was necessary to calculate these contributions which was done in [3], [4], [5]. It turned out that indeed they can give rise to effects of comparable size. However, it was shown that the normal component of the polarization vector is insensitive to rescattering effects and provides a direct measure of the strong coupling constant.

Higgs potential

Since the top quark is an exceptionally heavy particle, it may play a crucial role in the generation of masses. In particular, the top quark system may be optimally suited for investigations of the Higgs sector of the Standard Model. Therefore, we calculated the influence of a scalar and pseudo-scalar Higgs boson on the top quark production cross section near threshold [6]. It turned out that this cross section is significantly modified by the presence of a Higgs potential, especially for a low Higgs mass as it is favored by recent data compilations.


A comprehensive presentation of the main results obtained during the workshops on physics at a next linear collider (see [5]) can be found in [7].

New developments

The calculations mentioned above are valid mostly to next-to-leading order in the top quark velocity. Meanwhile there has been considerable theoretical progress, so that now the NNLO corrections to top quark pair production in the threshold region are available. The corresponding results showed a surprisingly strong dependence on the renormalization scale and have lead to interesting new ideas. For a review and a list of references see the paper on "Top-Anittop Pair Production close to Threshold: Synopsis of recent NNLO Results" [ref].


[1] R. Harlander, M. Jezabek, J.H. Kühn, T. Teubner,
Polarization in Top Quark Pair Production near Threshold,
Phys. Lett. B346 (1995) 137-142.
[2] R. Harlander M. Jezabek, J.H. Kühn, T. Teubner,
Polarization in Top Pair Production and Decay near Threshold,
Proceedings of the Workshop "Spin 94", Bloomington, IN, Sept. 1994, pp. 675-680.
[3] R. Harlander, M. Jezabek, J.H. Kuehn, M. Peter,
Top Quark Polarization in Polarized e^+e^- Annihilation near Threshold,
Z. Phys. C73 (1997) 477-494.
[4] M. Jezabek, R. Harlander, J.H. Kühn, M. Peter,
Polarized Top Quarks,
Proceedings of the workshop on Physics and Experiments with Linear Colliders, Morioka-Appi, 1995, pp. 436-446.
[5] R. Harlander, M. Jezabek, J.H. Kühn, M. Peter,
Top quarks at a polarized linear collider,
Proceedings of the workshop on e^+e^- collisions at TeV energies: the physics potential, part D, Annecy, Gran Sasso, Hamburg, 1995, pp. 5-20, ed. P. Zerwas.
[6] R. Harlander, M. Jezabek, J.H. Kühn,
Higgs effects in top quark pair production,
Acta Phys.Polon. B27 (1996) 1781-1788.
[7]ECFA/DESY LC Physics Working Group: E. Accomando, et al.,
Physics with e^+e^-Linear Colliders,
Phys. Rept. 299 (1998) 1-78.

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