II. Institute for Theoretical Physics
In the center of our research work is the theoretical investigation of the characteristics (e.g. masses, couplings, discrete symmetry characteristics, total and partial decay widths etc.) of elementary particles and their connection conditions (e.g. positronium, corpse, D and b-b-Mesonen, charmonium, bottomonium etc.), as well as their interactions during impact processes at high energies (e.g. electron-positron destruction, lepton-nucleon, nucleon-nucleon and photon-photon dispersion etc.) in the context of the standard model and its extensions (e.g. fourth fermion generation, supersymmetry, models with Majorana neutrinos or other exotic leptons, scenarios without Higgs bosons, effective theories with abnormal couplings etc.). The high accuracy of previous, current and planned particle physics experiments makes the inclusion of quantum corrections in the theoretical forecasts indispensable in many cases.
The Higgs sector of the electroweak interaction is so far almost unexplored from the experimental view. The production of reliable forecasts for production and decay processes of Higgs bosons therefore forms an emphasis of our research work. The treatment of lighter Higgs bosons can be simplified with the help of suitable low-energy theorems. On the other hand, the goldstone boson equivalence theorem offers a useful aid to the description of heavy Higgs bosons.
The hadronisation of quarks and gluons takes place in the energy region below approximately 1 GeV and is therefore a non-perturbative phenomenon, which is described in the context of the QCD parton model by phenomenological fragmentation functions. These know for example by global fits to precise e+-e- data with different energies to be determined. From the scale breaking occurring here the strong coupling constant αs can be extracted. The fragmentation functions are universal due to the factorizing theorem of QCD and find therefore various applications within the range of the inclusive hadron production.
The possibility opens recently in the context of the nonrelativistic QCD (NRQCD) a suggested factorizing theorem of describing Charmonium and Bottomonium by means of a formal separation from perturbative and non-perturbative phenomena systematically. A spectacular success of this approach was the fact that the production rate of J/ψ-mesons, measured at the Tevatron, which was more than one order of magnitude higher than the previous theoretical expectation in the color singlet model, now could be explained by the inclusion of so-called colour-octet processes. On the other hand, the inelastic J/ψ photoproduction in the quasi-elastic limit resulted in a significant discrepancy between the HERA data and the theoretical forecast at leading order in the case, which is called occasionally Tevatron-HERA anomaly. Experimentum crucis for the factorizing hypothesis of the NRQCD the measurement of the polarization of J/ψ-mesons is in the direct hadroproduction, which should be completely transverse for sufficiently large transverse momenta.
Since the top quark disintegrates because of its large mass, before it can hadronise, the production can be described by top quark pairs in the threshold region in the context of the NRQCD completely without non-perturbative parameters and thus in principle arbitrarily exactly. However, the appropriate perturbative expansion seems to converge only very slowly, so that now also the consideration of corrections of the order αs3 lines up.
Apart from the treatment of specific particle physics processes we concern ourselves also with a set of conceptional questions of the quantum field theory, e.g. subtraction regulations for dispersion relations for the computation lepton sector extended amended by ultraviolet-divergent Feynman amplitudes, low-energy theorems for scalar fields, calibration-independent definition of the mass, the partial and total decay width and the Cabibbo-Kobayashi-Maskawa matrix and/or its counterpart in theories with and/or.
Our future plans include among other things the following research projects:
* The scale behavior of αs is to be examined at third order in the momentum subtraction scheme and be compared with the appropriate behavior in the ms scheme.
* The production of charged Higgs bosons as well as top and bottom quarks with TESLA is to be treated in the supersymmetric standard model with consideration of strong quantum effects.
* The production of charmonium and bottomonium by hadroproduction at the Tevaton and at the LHC, by photo production and low-inelastic dispersion with HERA and by two-photon dispersion with LEP 200 and with TESLA is to be treated in the context of the parton model using the factorizing theorem of the NRQCD at next-to-leading order. In particular the Tevatron-HERA anomaly is to be cleared up and the existence of the colour-octet mechanism in nature to be examined.
* The excitation curve of the top quark pair production with TESLA is to be predicted in the NRQCD at third order in αs.