# 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/m_{b} 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