The main topics of our research are the physics of elementary particles, quantum field theory and some aspects of cosmology.

General information on particle physics in german can be found here

The focus of our curent research is on the following topics:

• Collider physics: Production of Higgs bosons, Top quarks, W-bosons
  
• Supersymmetry
• Flavour physics, B-physics
• Effective field theory
  
• Cosmology
  

Werner Bernreuther, Michael Krämer

The Higgs mechanism is a cornerstone of the Standard Model (SM) and its supersymmetric extensions. The masses of the fundamental particles, electroweak gauge bosons, leptons, and quarks, are generated by interactions with Higgs fields. The search for Higgs bosons is thus one of the most important tasks for high-energy physics and is being pursued at current and future collider experiments.

We pursue research to improve cross-section predictions for Higgs-boson production at the LHC within the Standard Model and, in particular, its supersymmetric extensions through the calculation of QCD, electroweak and supersymmetric higher-order effects. One current focus is the production of supersymmetric Higgs bosons in association with heavy quarks, an important discovery channel which allows to directly determine the Higgs-Yukawa couplings.

Non-standard Higgs bosons, if they exist, can have unusual properties. For instance, their interactions with quarks and leptons can violate the CP symmetry, in a way that is completely different from the Kobayashi-Maskawa mechanism of the Standard Model. Higgs-sector CP violation is a fascinating possibility, with implications on the physics of the early universe. Another line of our research is to devise and investigate strategies how to explore the CP properties of Higgs bosons, resp. how to search for Higgs-sector CP violation at the LHC.

Martin Beneke, Werner Bernreuther

The top quark is the heaviest fundamental particle known to date. Although it weighs more than about 180 hydrogen atoms (172 GeV/c²), it behaves like an elementary particle, at least at distance scales probed so far by experiments. The top quark is special among the known quarks: once produced it immediately decays without forming top-mesons or -baryons. Thus, it provides the unique opportunity to study the interactions of a bare quark. At the moment such heavy objects can only be produced at the Tevatron collider at Fermilab near Chicago and (from 2008 on) at the Large Hadron Collider (LHC) at CERN near Geneva.

The LHC will be a top-quark factory: millions of these quarks will be produced at this collider per year -- either together with their antiparticles or singly. This will allow to explore the interactions of this quark in great detail -- provided adequate modeling is available. The research of Werner Bernreuther's group is concerned with theoretical predictions of top quark production and decay, at the LHC and the Tevatron, within the standard model of particle physics and also more speculative models that describe possible new interactions. Aspects of this research include investigations of the role the top quark can play in exploring the ``mechanism of mass generation'', and theoretical analyses of top-quark spin effects.

At a future high-energy electron-positron collider (such as the International Linear Collider) a top-quark and its anti-particle can be produced with very small relative velocity. Under the influence of the strong interaction they can form a bound state similar to the Hydrogen atom, but 10 million times smaller. Because of the short life-time of the top quark, this "toponium" bound-state decays rapidly. According to the uncertainty relation this corresponds to a broad resonance that can be scanned to measure the mass of the top quark with high accuracy.
A precise theoretical description of this process requires the consideration of high orders in the pertubative expansion and the development of new methods to take quantum-field theoretical effects correctly into account.

A more detailed discussion of our work on top quark physics (in German) can be found in "RWTH-Themen (pdf)" For an overview of top-quark physics LHC see a recent review.

Martin Beneke, Michael Krämer

The relations among the masses and couplings of gauge bosons allow decisive tests of the electroweak interactions, complementing direct collider searches for Higgs bosons and physics beyond the Standard Model. The W mass, in particular, provides an important input for constraining the Higgs boson mass from electroweak precision data.

Electroweak radiative corrections to the W-boson production cross section and distributions need to be included to match the experimental accuracy expected from W-mass measurements at the Run II of the Tevatron and at the LHC. A recently completed project has addressed the impact of photon-induced processes, multi-photon emission and the MSSM electroweak corrections on single-W production. Future work is planned to study the impact of higher-order electroweak corrections on the W-mass determination.

At a future International LInear Collider the experimental precision of measurement of the W-mass could be further improved compared to the LHC by measuring the rise of the cross section for the production of W pairs near the production threshold. An ongoing project is concerned with the precise theoretical description of this process using methods from effective field theory.

Michael Krämer


In the last 20 years, the Standard Model has been successfully tested. The only remaining undiscovered particle is the Higgs boson. Even if the Higgs boson is discovered, there are various reasons to expect that the Standard Model needs to be extended; most notably because of the hierarchy problem, the existence of dark matter, the mechanism of baryogenesis and because gravity is not included in the SM. One class of theoretical models attempting to solve the problems of the SM is based on the idea of supersymmetry (SUSY), with a new superpartner for each known particle. The lightest SUSY particles are expected in the mass range 100-1000 GeV and can be discovered at the LHC.

The Interpretation of the experimental results in terms of supersymmetric models requires precise theoretical predictions. A comprehensive project has focused on the calculation of next-to-leading order QCD corrections to the production of MSSM particles at the Tevatron and at the LHC; the associated production of squarks or gluinos with gauginos is currently under study. The improved cross-section predictions play an essential role in ongoing SUSY searches by the CDF and D0 collaborations at the Fermilab Tevatron and, in the case of discovery, will be exploited to determine the masses of the SUSY particles.

Precise information on sparticle masses (or mass differences) and sparticle spins can be obtained at the LHC from the measurement of cascade decays. In a recent project we have considered the impact of QCD and electroweak corrections on differential distributions and kinematic correlations in supersymmetric cascade decays. A proper theoretical understanding of these processes is crucial to determine the parameters of supersymmetric or other new physics models at the LHC with the expected high experimental accuracy.

Martin Beneke

"Flavour" refers to the property of quarks that distinguishes them as "up" (u), "down" (d), "charm" (c), "strange" (s), "top" (t) and "bottom" (b) quarks, and is ultimately related to the couplings of the quarks to the Higgs field. (A similar distinction applies to the leptons.) In present-day experiments "flavour" manifests itself in widely different quark masses from a few MeV/c² (up, down) to 175 GeV/c² (top) and transitions of quarks of one flavour into another, such as c → s, b → u of widely varying strengths.

Flavour physics has several intriguing features:

• Transition stengths between flavours (embodied in the Cabibbo-Kobayashi-Maskawa matrix) are strongly hierarchical.
• Violations of CP symmetry (parity+charge conjugation), implying microscopic non-invariance under time reversal have so far only been observed in flavour-changing transitions of the quarks.
• Flavour changing transitions between quarks of equal electric charge, such as b → d, are absent in the Lagrangian and occur only through quantum fluctuations.

These features also imply that flavour-changing and CP-violating processes are rather sensitive to new interactions and particles even if they have masses significantly larger than the W boson (80 GeV/c²) of the weak interaction. Decays of B mesons -- bound states of a bottom antiquark and a light quark (u, d or s) -- are particularly revealing and have been studied experimentally in great detail in two dedicated experiments since 2000.

Theoretical research in flavour and B-meson physics focusses on calculating observables that exhibit the characteristic features of the underlying flavour-violating interaction, as well as the signatures of potential new interactions and particles (mainfesting themselves in quantum effects) that would imply a failure of the standard theory. In addition, since B mesons are bound by the strong interaction, a crucial aspect of this research is understanding and calculating how the strong interaction affects the decay distributions, CP asymmetries etc. in order to deduce the structure of the microscopic quark interactions from the observations of B-meson decays.