On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

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One of the most fascinating insights of the last fifteen years regarding new physics is
that, due to a conjecture by Maldacena, one can describe concepts in strongly coupled
theories by a five dimensional (5D) model, based on a compact extra dimension with
an unfactorizable Anti-de Sitter metric, the Randall Sundrum (RS) model. It adds
to the fascination, that the concept of partial compositeness is automatically implemented
in these models by making the quarks 5D fields.
In the first half of Chapter 2, this duality will be motivated, and the two different ways
of describing the same physics will be used for a deeper understanding of the analyses
in the latter parts of the thesis. As a new result, a dual interpretation of the small
momentum expansion of the 5D propagator for gauge bosons will be presented there.
The rest of Chapter 2 will be devoted to the technical formulation of the fundamentals
of the RS model. Important in this context is the geometrical explanation of how the
suppression of FCNCs is realized by the RS-GIM mechanism.
Chapter 3 includes an update on the RS contributions to electroweak precision measurements,
including the consequences of a new global fit for the oblique parameters
S and T and the impact of new theoretical results on the conclusions drawn from the
Z ¯ bb for the RS parameter space. The effectiveness of the RS-GIM mechanism will be
demonstrated on the basis of observables connected to the decay of Bs and Bd mesons
to muons and Bs − ¯ Bs mixing. The heavy b quark makes them well suited to test the
limits of the RS-GIM mechanism, because the suppression of FCNCs becomes less effective
the heavier the involved flavors are. While it turns out that these observables
generically agree even with the small errors from recent LHCb measurements, one
finds that the CP violating quantity in K − ¯ K mixing, ɛK will induce a fine-tuning
of the order of one percent if no additional suppression of the RS contributions is
implemented.
This is rooted in a chiral enhancement and large renormalization group effects, and
therefore it is a largely model independent problem for any new physics with vector
couplings to quarks. In RS models it is the only observable which needs fine-tuning
and this issue has therefore been termed the “RS flavor problem”. After the explanations
proposed in the literature are collected and interpreted in the dual theory, a
novel solution based on an extension of the color gauge group will be presented and
discussed in detail. This solution is especially attractive, because it preserves the anarchical
flavor structure of the Yukawa matrices, which is essential in order to explain
the hierarchies in quark masses and mixings. In the extended model the aforementioned
observables will be computed, as well as the cross section for direct detection
of the involved new resonances at the LHC.
The fact that the size of the couplings of RS gauge fields to quarks depends on the
the quark mass, makes top observables an ideal candidate to look for effects of the RS
model. Possible contributions to the forward-backward asymmetry measured by CDF
and DØ , as well as the agreement with the total cross section in top antitop pairs
will therefore be studied in Chapter 4. In a model-independent analyses, the impact
of general new physics in the s and t channel will be computed and in particular the
effects in the RS model as well as in the RS model with the proposed extension will
be shown.
Concluding remarks and an outlook on future work in Chapter 5 will complete the
thesis.