On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

5 Conclusions
Whenever the Large Hadron Collider begins a new run, the first sign of physics beyond
the Standard Model could be revealed, and this becomes more likely with the
recorded luminosity growing exponentially. However, as long as the mass gap between
the Higgs and new physics resonances grows, theories which naturally predict
new particles with masses around the electroweak scale become more and more tuned.
In the Randall Sundrum model and its dual description by a strongly coupled theory,
new resonances are only expected in the multiple TeV range, and, due to the RS GIM
mechanism, couple more or less exclusively to top quarks. As was shown in Chapter
3 and 4, this can explain why they have stayed below the radar of the ever increasing
exclusion limits of the LHC.
In addition, the RS model arguably provides the best explanation of flavor we have
today. Both the absence of FCNCs as well as the structure of the CKM matrix can be
explained based on the fact that the quarks are localized differently along the extra
dimension, which can be interpreted as a mixing with new composite degrees of freedom.
As a result, flavor bounds are suppressed by orders of magnitude compared to
general new physics without such a protection mechanism. This becomes apparent in
the generically good agreement of the RS predictions for the flavor changing processes
studied in Chapter 3. We find that both the contributions to Sψφ and ∆Γ/Γ in the
Bs − ¯ BS system as well as the branching ratios B � Bs → µ + µ −� and B � Bd → µ + µ −�
do not exceed the experimental expectations and can even lead to an improved agreement,
even if the contributions from the custodially protected model are taken into
account.
It is thus remarkable that the RS-GIM mechanism leads to a broad agreement with
measurements in the flavor sector, except for indirect CP violation in the Kaon system,
which requires one percent fine-tuning despite the good suppression for the light
flavors involved. It was extensively discussed how this problem arises and how it can
be ameliorated following the approaches proposed in the literature. Most of these
solutions modify the Yukawa couplings or constrain the localization parameters for
the quarks, whose randomness is essential for the explanation of the flavor structure
of the CKM matrix.
A new solution has been motivated by the observation, that the dangerous contributions
to ɛK arise almost exclusively because of the exchange of color-charged resonances,
the KK modes of the gluon. The extension of the strong interaction gauge
group to a SU(3)D ×SU(3)S surprisingly cancels these contributions independently of
the imposed boundary conditions for the bulk fields and the choice of gauge couplings
for the two subgroups.
Invoking the AdS/CFT duality, in a detailed analysis the part of the 5D gauge boson
propagator which is responsible for the excessive contributions could be related to
the purely composite contribution of the mixed elementary-composite propagator in
Chapter 2. This allowed for a deeper understanding of the cancellation, that protects
ɛK and ultimatively leads to an RS model in agreement with all bounds from
the quark flavor sector. In Chapter 3, the thorough implementation of this model is