On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

176 Chapter 5. Conclusions
described, which unfortunately implies a baroque Higgs sector in order to realize the
Yukawa couplings on the IR brane, that does not lead to a predictive mass spectrum
for the new scalar states due to the vast number of free parameters in the potential.
The impact of the RS model with and without the extended color gauge group on
flavor diagonal processes, namely electroweak precision observables, has also been
studied in Chapter 3. It turns out that the recent updates on the fit to the Z → ¯bb pseudo observables put very severe constraints on the couplings of the Z or induces
a large KK scale. Adopting a custodial protection in order to prevent this, which is
also the best way to suppress large contribution to the oblique T parameter, however
reintroduces a tension with the contributions from the resonances charged under the
extended color gauge group to ɛK.
If the flavor problem of the RS model is explained by the mechanism introduced in
this thesis, this tension must be addressed by either a different suppression of contributions
to the electroweak precision observables, or by a more inventive realization
of the scalar sector. In this context it should be mentioned that during the completion
of this thesis, a paper has been published, whose authors also find only a minor
improvement for the custodial model with extended color gauge group [244]. The
bound derived there comes from the loop induced direct CP violation in Kaon mixing
measured by ɛ ′ /ɛK and is due to the exchange of the new scalar degrees of freedom
and therefore also rooted in the extended Higgs sector.
The last chapter focuses on the question of whether the question whether the observed
forward-backward asymmetry at the Tevatron could be explained by new physics, with
the emphasis on the RS model and its extensions. Because the KK gluons have almost
pure vector couplings to quarks, one finds generically larger contributions to the total
inclusive cross section than to the asymmetric cross section in the RS model. This
leads to a negative correction to A t FB
, that is defined as the ration of the two. The
measurement however suggests an enhancement. This result could be generalized to
all new physics with primarily vectorial couplings and its robustness under the inclusion
of interference effects with NLO QCD diagrams has been confirmed.
The almost axial vector couplings of the other linear combination of gauge fields from
the SU(3)D × SU(3)S → SU(3)C breaking in the extended model does already lead
to a suppression of the asymmetric cross section and consequentially leads to an even
worse fit, which is in agreement with the general expectations of resonances with axial
vector couplings. In contrast to ɛK, A t FB
is very sensitive to the boundary conditions
of the color-charged 5D gauge bosons and it could be shown that the BCs imposed
by direct detection bounds and the Yukawa couplings guarantee that the effects are
always suppressed by the localization of the up quarks and are therefore rendered very
small.