Projeck Area C: Heavy Quarks and Flavour Physics

The origin of the pattern of fermion masses, flavour mixing and CP violation is one of unsolved puzzles in theoretical particle physics. Important progress can be expected, as far as the quark sector is concerned, from the investigation of quarks of the third family, the bottom and the top quark.

The analysis of flavour-changing interactions in the standard model (SM) and the potential discovery of new such interactions is complicated by the strong interaction of the quarks, which is always present. Thus, despite the weak origin of flavour, precise calculations of QCD effects are an indispensable prerequisite in heavy quark physics. In the case of bottom quarks a combination of perturbative and non-perturbative methods is usually applied for this purpose. Production and decay of top quarks, on the other hand, can be calculated in perturbation theory due to the large mass and width of this quark. Nevertheless, methods beyond standard fixed-order perturbation theory are often required, such as for non-relativistic situations, for the description of the top as a resonance, and for the efficient treatment of massive quark jets. Project group C combines key topics of this CRC/TR, ``Multi-loop calculations'' and ``Lattice gauge theory'', and applies them to flavour and heavy quark physics.

To achieve its goals -- precise control of the strong interaction and the study of weak and new interactions -- Project group C contains the five projects (four in the next funding period):

C1 Strong interaction effects in B-Meson decays
C3 Threshold production of top quarks and other heavy particles
C4 Top-quark physics at collidern
C5 Multi-loop calculations with heavy fermions in the SM and MSSM
C6 Flavour physics beyond the Standard Model

Precise calculations of strong interaction effects in B-meson decays are the focus of project C1. On the one hand, heavy quark effective theory is implemented on the lattice. Based on the results in the previous funding periods, it will now be possible to study properties of b quarks including 1/mb corrections, non-perturbative renormalisation, and with dynamical quarks reaching small light-quark masses. On the other hand, non-leptonic two-body decays, which are inaccessible with lattice methods, are considered on the basis of QCD factorisation. This part combines factorisation studies with loop calculations (e.g., the direct CP asymmetries at next-to-leading order) and phenomenological studies. The project as a whole contributes to tests of the flavour sector of the SM by sharpening the theoretical predictions in the SM.

The physics of the top quark near the pair production threshold, and more generally, the interactions of heavy particles moving at relative velocities much smaller than the speed of light, is the subject of project C3. Calculations are performed that will allow to determine the mass of the top quark with an error smaller than 100\MeV at an e+ e- collider, together with its width and Yukawa coupling. The project now includes top-quark pair production and super-particle pair production at hadron colliders as well as studies of dark matter pair annihilation. The common features of these problems are that they can be efficiently solved with non-relativistic effective field theory, requiring a combination of multi-loop matching calculations with resummations in the effective theory.

Project C4 is devoted to precise predictions for the production and decay of heavy quarks, primarily top quarks, in high-energy hadron and e+ e- collisions. A new focus of this project in the third funding period is on improving the predictions for single-top production at the LHC to next-to-next-to-leading order including differential distributions. As far as methods are concerned this requires the construction of a NNLO subtraction algorithm for infrared divergences in processes containing massive fermions. This is relevant as well to the calculation of production of Q barQ pairs at NNLO in e+ e- collisions, also pursued in this project. Furthermore, the influence of new interactions and Higgs bosons decaying into top-quark pairs are studied, since they provide information on the electroweak symmetry breaking sector.

The heavy quarks of the SM manifest themselves not only in their direct production and decay, but also indirectly in virtual (loop) effects. These virtual effects are the subject of project C5. Here, the decay rate of the b -> sg transition will be computed at three-loop order in the two-Higgs doublet model, extending the known result in the SM. The virtual effects of heavy quarks on Higgs production and decay, and Higgs boson masses in the minimal supersymmetric standard model (MSSM) will be considered. The project also contains the calculation of decoupling relations for MS parameters between the SM and the MSSM and grand unified theories at high loop orders, which allow the precise extrapolation of couplings from low to high energies. An important aspect of the project is the development of the computer-algebra techniques to solve the problems described above and related ones.

Finally, project C6 was devoted specifically to flavour physics beyond the SM. The motivation for this is that generic extensions of the SM should yield large flavour-changing neutral currents compared to those existing in the SM. Since the nature of new physics is unclear, new physics studies are either based on general parameterisations of operators that emerge after integrating out new particles or concrete models which exhibit interesting effects. In this project, a grand-unified model, which has connections with lepton flavour physics, has been investigated, as well as the MSSM at large value of the ratio of Higgs vacuum expectation values, tan beta.

Last Changed: 8th June 2011