Challenges for New Physics in the Flavour Sector

In these proceedings I present a personal perspective of the challenges for new physics (NP) searches in the flavour sector. Since the CKM mechanism of flavour violation has been established to a very high precision, we know that physics beyond the Standard Model can only contribute sub-dominantly. Therefore, any realistic model of physics beyond the Standard Model (SM) must respect the stringent constrains from flavour observables like $bto s gamma$, $B_stomu^+mu^-$, $Delta F=2$ processes etc., in a first step. In a second step, it is interesting to ask the question if some deviations from the SM predictions (like the anomalous magnetic moment of the muon or recently observed discrepancies in tauonic $B$ decays or $Bto K^*mu^+mu^-$) can be explained by a model of NP without violating bounds from other observables.

Supersymmetric renormalisation of the CKM matrix and new constraints on the squark mass matrices

We compute the finite renormalisation of the Cabibbo-Kobayashi-Maskawa (CKM) matrix induced by gluino-squark diagrams in the MSSM with non-minimal sources of flavour violation. Subsequently we derive bounds on the flavour-off-diagonal elements of the squark mass matrices by requiring that the radiative corrections to the CKM elements do not exceed the experimental values. Our constraints on the associated dimensionless quantities delta^{d LR}_{ij}, j>i, are stronger than the bounds from flavour-changing neutral current (FCNC) processes if gluino and squarks are heavier than 500 GeV. Our bound on |delta^{u LR}_{12}| is stronger than the FCNC bound from D-D-bar mixing for superpartner masses above 900 GeV. We further find a useful bound on |delta_{13}^{u LR}|, for which no FCNC constraint is known. Our results imply that it is still possible to generate all observed flavour violation from the soft supersymmetry-breaking terms without conflicting with present-day data on FCNC processes. We suggest that a flavour symmetry renders the Yukawa sector flavour-diagonal and the trilinear supersymmetry-breaking terms are the spurion fields breaking this flavour symmetry. We further derive the dominant supersymmetric radiative corrections to the couplings of charged Higgs bosons and charginos to quarks and squarks.