Publikationsserver der RWTH Aachen University
Vorwärts-Rückwärts-Asymmetrie bei massiven Quark-Jets
Abstract
dc:descriptionIn this thesis, we calculate the scattering amplitude of the process e^+ e^- --> Q Qbar up to the second order in the QCD coupling constant (alpha_s) and in the lowest order of the electroweak couplings. For this purpose, we evaluate the (gamma* Q Qbar)- and (Z* Q Qbar)-vertex functions. These are important contributions for the fully differential description of the production of a heavy Q-Qbar pair (Q=c,b,t) in the reaction e^+ e^- --> Q Qbar + X to order alpha_s^2. The fully differential description is needed for the calculation of physical observables, as for example forward backward asymmetries (A_FB). We paramterize the vertex functions through dimensionsless, CP-concerving form factors. The calculation of the form factors to the two-loop vertex functions with one internal mass is done in dimensional regularization. The arising two-loop integrals are evaluated with the help of integration-by-parts identities, Lorentz-invariance identities and the Laporta-algorithm. The renormalization of the ultraviolett divergencies is done in a hybrid renormalization scheme: the renormalization of the QCD coupling constant and of the gluon wave function is done in the MSbar-scheme, the renormalization of the quark wave function and of the quark mass is performed in the on-shell scheme. In the second order of alpha_s, we have two different types of amplitudes for which we have to use different schemes for the treatment of the matrix gamma_5 in dimensional regularization. The consistency of the results is checked with the help of Ward-identities. We evaluate the form factors in physically interesting limits. In the asymptotic limit the axial-vector form factors and the vector form factors become identical. This is due to helicity conservation of the gauge boson vertices in that limit. The results for the threshold limit can be compared with already existing calculations. With the help of the form factors, we calculate the two-parton contribution to A_FB, defined with respect of the heavy quark direction. For two partons in the final state the heavy quark direction is identical to the thrust direction. The two-parton contribution has no infrared divergencies in first and second order of alpha_s, and can therefore be safely evaluated. We perform a numerical evaluation for bottom-quarks on the Z-resonance and for top-quarks for center-of-mass energies from the t-tbar-production threshold up to 500 GeV.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2005
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Heinesch, Roland
- Contributors dc:contributor
-
- Bernreuther, Werner
Subjects
dc:subject × 5Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger