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Particle Phenomenology


Discrete symmetries

O.Nachtmann, T.Bergmann, A.Utermann
The discrete symmetries parity (P), charge conjugation (C) and time reversal (T) are all broken individually in nature, however, their product is conserved (CPT theorem). In atomic physics P-violation effects are described by the exchange of Z-bosons between the electrons of the shell and the quarks in the nucleus. We study energy shifts and geometrical (Berry-) phases as new manifestations of this interaction. The possibilities to observe these effects with the help of an atomic-beam interferometer are studied.

Non conservation of the combined symmetry CP of charge conjugation and parity has been observed experimentally in the neutral K-meson system and recently also in the neutral B-meson system. We study CP-odd effects in high energy reactions as e.g. in Z-decays and electron-positron collisions at a centre of mass energy of about 100 to 800 GeV. In this way information about possible extensions of the standard model of elementary particle physics can be gained (multi-Higgs-models, supersymmetric models, etc.). In this framework a future international linear collider (ILC) with a centre of mass energy of 500 GeV and more is of special interest (see also anomalous gauge-boson couplings at an ILC ).

Some publications

Anomalous gauge-boson couplings at an ILC

O.Nachtmann, A.Utermann
In the Standard Model (SM) of particle physics the couplings between the gauge bosons of the electro-weak interaction are fixed by the requirement of renormalisability. The testing of these couplings is an important goal for recent and future experiments, especially at a future international linear collider (ILC). We develop theoretical methods to probe these couplings in an optimal way and we investigate the influence of anomalous effects on precision observables.

Such anomalous effects can be tested also at the coming pp collider LHC. In the framework of a LHC seminar, we would like to stimulate discussions between theoretical and experimental physicists concerning the goals one would like to achieve at the LHC. Let us mention that, due to the large energies, the LHC is of fundamental interest concerning investigations in the framework of the theory of high-energy scattering and offers new possibilities for the development of our knowledge of the non-trivial vacuum structure .

Some publications

Theory of high-energy scattering

  O.Nachtmann, A.Utermann

Also involved is the Nuclear Physics Group (H.J.Pirner).

The theoretical description of hadron scattering at high energies and small momentum transfers in the framework of Quantum Chromodynamics (QCD) is the theme of this research. Neither perturbation theory nor the numerical methods of lattice gauge theory are suited for treating this problem.

Starting from the functional integral of QCD, we have developed a way to represent the relevant scattering amplitudes as expectation values of light-like Wegner-Wilson loops. The calculation of these expectation values can be performed using non perturbative methods, especially in the framework of the model of the stochastic vacuum.

A further area of research in high-energy scattering, which is investigated in our group, is the regime of small values of the Bjorken variable x which is measured in deep inelastic scattering at HERA.

Some publications

Consequences of the non-trivial vacuum structure

O. Nachtmann
In a non-linear theory like QCD the state of lowest energy (the vacuum) most likely has a non-trival structure. This may be the key for a phenomenological understanding of several interesting and surprising effects in QCD, both in low energy physics (spectroscopy) and in high energy scattering. For many applications the vacuum structure is modeled in a Gaussian stochastic model. It provides an analytical tool for calculating processes not (yet) treatable by other means.

Some publications

Weak Decays

  B. Stech
The fundamental parameters of the Standard Model concerning quarks can be only extracted indirectly from hadronic properties. Several methods such as chiral symmetry, QCD sum-rules, Heavy-Quark Effective Theory, and quark models are applied in order to determine the elements of the Cabbibo-Kobayashi-Maskawa matrix, to test the Standard Model, and to obtain a detailed understanding of numerous experimental results.

Neutrino Masses and Oscillations

B. Stech, C.Wetterich
The new experiments on solar, atmospheric and accelerator based neutrino physics provide evidence for neutrino oszillations. An interesting mass spectrum of the 3 kinds of neutrinos can be expected. In particular, the observation of an intriguing mixing pattern of the neutrinos with two large mixing angles and a third very small one is of great interest. These results give the first experimental proof for the necessity of extending the standard model of particle physics. In the simplest extension one assumes the existence of very massive right handed neutrinos as predicted in grand unified theories (GUT's). These neutrinos are essential for the tiny masses of the light neutrinos and may determine some properties of the early universe. Of particular interest for us is the possibly close connection between the spectrum and mixings of quarks and charged leptons with those of neutrinos. These kind of problems are at present the most thrilling parts of particle physics. In some of our attempts we make use of the challenging E6 grand unified symmetry. Based upon our earlier work on neutrino physics and GUT's we try to understand the masses and mixings in the frame work of GUT's supplemented by additional generation symmetries. Our goal is to construct a consistent unified theory in which all quantities in the mass matrices of quarks and leptons are explained by the breaking of the combined GUT and generation symmetry. The large power like hierarchy of charged fermions and the presumably less strong hierarchy of the neutrinos give us a first hint for a description in terms of a small universal parameter. Forthcoming results of experiments will shed further light on neutrino properties and will allow us to test existing models and to restrict and improve the present ideas. The heavy neutrinos and other new particles involved play a testible role in the cosmology of the early universe.

Some of our very early publications connected to neutrinos and GUT's :

  • M. Magg, C. Wetterich, Phys. Lett. 94B(1980) 61;
  • C. Wetterich, Nucl. Phys. B187 (1981) 343; Nucl. Phys. B279 (1987) 711;
  • B. Stech, in: Flavour Mixing in Weak Interaction, Ed.L.L. Chau, Plenum Press,V20 (1984) 735;
  • J. Bijnens, C. Wetterich, Nucl. Phys. B292 (1987) 443

More recent publications by us on neutrinos and GUT's :

  • C. Wetterich, hep-ph/9812426, Phys. Lett.B 451 (1999) 397;
  • B. Stech, hep-ph/9905440, Phys.Lett.B 465:219-225, 1999;
  • B. Stech, hep-ph/9909268, Baltimore 1999, Neutrinos in the new millennium (295-303);
  • B. Stech, hep-ph/0006076, Phys.Rev.D62:093019, 2000;
  • B. Stech and Z. Tavartkiladze, hep-ph/0311161, Phys.Rev.D70:035002, 2004.