# Subproject C1: Strong interaction effects in *B* meson decays

Project Leaders:

Dr. Rainer Sommer, DESY Zeuthen

*B* physics experiments of the past decade have confirmed the
Kobayashi-Maskawa mechanism of CP violation in its essence,
but leave open the possibility of small deviations, or deviations
in specific flavour-changing transitions. *B* decays with
hadrons in the final state are numerous and well suited to
investigate this question. However, the presence of hadrons
implies that strong interaction effects must be addressed
together with the CP-violating and flavour-changing phenomena
of interest.

This project is devoted to a quantitative understanding of the strong
interaction in *B* decays with perturbative (QCD factorization) and
non-perturbative (Heavy Quark Effective Theory on the lattice) methods.
The focus is on exclusive decays: leptonic, semi-leptonic
as well as non-leptonic two-body.

**Heavy Quark Effective Theory**

The non-perturbative renormalization of the theory has been tested and applied
in the quenched approximation.
Remaining work concerns components of the weak flavour currents other than the
time-component of the
axial current used for f_{B}. The emphasis is on the application with dynamical
fermions
in collaboration with project B2. For controlling the chiral extrapolation, the
leading low energy
constant in the heavy meson chiral Lagrangian is computed and then applied in
fits describing the
light-quark mass dependence of observables. Decay constants of *B* and B_{s},
the b-quark mass and
form factors of semileptonic decays (close to q^{2}_{max}) are being computed.
All numerical computations are carried out at a series of lattice spacings to
be able to reach continuum results
by a controlled extrapolation.

**Exclusive B-Meson Decays**

We concentrate on the following areas:

- Radiative corrections:

calculation of the non-leptonic decay tree and penguin amplitudes in QCD factorization at next-to-next-to-leading order both for the vertex term and for spectator-scattering . -
*B*decay phenomenology:

Re-analysis of decays to two pseudoscalar and one pseudoscalar and one vector meson given the new input from item a; determination of the CKM angle γ. Combination of the QCD factorization approach with SU(3) symmetry for non-leptonic and electroweak penguin decays. -
Conceptual studies:

investigation of the factorization properties of 1/m_{b}suppressed penguin amplitudes, and the calculation of one-loop corrections to them; investigation of ``endpoint divergences'' that prevent the straight-forward application of soft-collinear effective theory to power-suppressed effects.

Last Change: 12th April 2012