Theoretical research

The Department of Theoretical High Energy Physics has long-time experience in all aspects of physics related to the experimental projects discussed above. This includes strong interaction both in the perturbative and the non-perturbative domains, re-summation techniques, renormalization group methods and evolution equations, matrix element calculations, effective field theories, and model building. This also includes phenomenology of different scenarios for supersymmetry, extra dimensions and other forms of "new physics". The researchers have also a worldwide recognized expertise in computer simulations, which is absolutely crucial for any comparison between theoretical ideas and experimental analysis. The Lund models and Monte Carlos are used in all high energy experiments.

Below is a sample of investigations that could be performed:

(a) Small-x and kT-factorization:
The description of hadronic final states at small x, e.g. within the framework of CCFM and the Linked Dipole Chain model.  Understand the dynamics of the soft non-perturbative region, which is enhanced by the running coupling constant, and the interplay between perturbative and non-perturbative effects. Try to make global fits to experimental data, both inclusive and exclusive, to obtain reliable parameterizations of the un-integrated parton densities. Apply the findings to make predictions for various observables at the LHC.
(b) Unitarity constraints:
Connection between saturation, diffraction and multiple interactions: Investigate the Abramovskii-Gribov-Kancheli cutting rules of Regge theory inside QCD, determining the applicability and limitations of the AGK rules for the relations between these phenomena. Investigate non-linear and colour-suppressed effects in the small-x evolution in the dipole formalism in transverse coordinate space. Implement the results in an event generator and compare with HERA, the Tevatron and make predictions for the LHC.
(c) Matrix elements and parton showers:
Develop models for combining fixed order tree-level and next-to-leading-order matrix element with parton showers, to improve our understanding of events with many hard jets. Compare with HERA and the Tevatron. Make predictions, mainly for QCD background, to signals of new physics at the LHC.
(d) Simulation of new phenomena:
Improve event generators for “new physics”, including production of new particles, e.g. different Higgs and SUSY scenarios. New models are frequently proposed, and to make these accessible to experimental testing, extensions to our current framework become necessary. As an example, some SUSY scenarios give multiscale showers, where coloured short-lived particles undergo sequential decays. The radiation pattern includes interference effects between the production and decay. In this and similar examples new patterns of perturbative and nonpertubative activity have to be studied.
(e) CP violation and Matrix Elements:
Improve estimates of asymmetries and widths necessary for CP violation measurements. This includes studies of the perturbative-non-perturbative interface and the interplay between high energy weak interaction phenomena and hadronic structure. Apply the findings to predict and/or explain observables in the flavour part of the LHC and other experimental programs.
(f) Theoretical and phenomenological aspects of electroweak interactions:
Studies of invariant functions of quark and lepton mass matrices and their applications to experiments. Mathematical structure of the quark and lepton mixing matrices and their parameterisation. Parameterisation of n-by-n unitary matrices and their applications to physical problems. Phenomenological aspects of CP violation and Higgs physics. Theoretical studies of axial anomaly and their physical implications.
(g) Phenomenological aspects of Large Extra Dimensions:
Study signals of gravitational scattering and mini black hole production at the LHC in the presence of Large Extra Dimensions. Improve the models for such processes and implement them in Event Generators. Compare the signals to Standard Model backgrounds.