on (a stack of) D-branes and their push-forwards to the target-space bulk.

LPTENS/04-46\
November 2004\

14 pt

Introduction
============

The Kondo effect \[1–6\], [*i.e.*]{} the screening of magnetic impurities by the conduction electrons in a metal, has played a key role in the early development of the renormalisation group. More recently it has also proved instrumental in unraveling the elegant structure of boundary conformal field theory. In modern language, the screening describes the RG flow between the Cardy states [@Cardy:1989ir] of the $\hat{su}(2)_k$ WZW model, where $k$ is the number of electron channels that participate in the process — see [@Affleck:1995ge; @Saleur:1998hq] for reviews and further references.

In string theory conformal boundary states correspond to D-branes, and this has revived interest in the problem from a different, more geometrical point of view.[^2] It has been appreciated, in particular, that the symmetric WZW branes wrap (twisted) conjugacy classes of the group manifold [@Alekseev:1998mc; @Felder:1999ka], that they are solutions of the Dirac-Born-Infeld equations [@Bachas:2000ik; @Pawelczyk:2000ah; @Bordalo:2001ec], and that they are classified by an appropriate version of K-theory \[16–20\].[^3] It was furthermore argued that the RG flow between different (stacks of) D-branes in these models is a process that lowers the energy by 