On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

Preface
Before the advent of the Standard Model,
physicists had become used to experiments
producing unexpected new particles or other
signposts to a new theory almost before the
chalk dust had settled on the old one. They have
been waiting 30 years for that to happen with
the Standard Model.
Gordon Kane
When Gordon Kane wrote this statement seven years ago, despite its success, one
crucial ingredient of the Standard Model (SM) was still missing, the Higgs boson. In
the meantime, the Large Hadron Collider (LHC) has started running, and essentially
rediscovered every known particle in the SM and nothing else, but one scalar resonance
at roughly 126 GeV. If this scalar is the Higgs, one can conclude from vacuum
stability bounds, that its mass nicely agrees with the SM as a complete theory of nongravitational
forces up to almost the Planck scale. At this point, its couplings are not
measured with an accuracy that would allow it to identify the SM Higgs with all its
predicted characteristics. However, one can already say that electroweak symmetry
breaking (EWSB), as it is described by the SM, is the correct low energy theory. The
desert of scales above the electroweak scale that the LHC begins to explore, lets one
wonder how many orders of magnitude this “low” energy description actually covers.
Thus, from an experimental point of view, the SM is a success. Theoretically, the LHC
measurements have made the questions left open by the SM only more pressing. This
thesis will address two of the most puzzling ones: What sets the electroweak scale,
and what is the reason for the hierarchies in the masses and mixings of the quarks?
The first question has been the driving force for building models of physics beyond
the SM for decades. In the SM, the electroweak scale is a free parameter and unstable
against radiative corrections. It must therefore be fine-tuned to a large degree in order
to agree with the experiment, unless some symmetry or mechanism protects it from
these radiative corrections.
In the first part of the Chapter 1, a survey of ideas that can solve this problem will
be presented. The emphasis will be put on theories which assume the Higgs to be a
strongly coupled bound state by some new force. If the LHC continues to set better
and better exclusion limits on new resonances, this explanation becomes attractive,
because the Goldstone bosons of QCD are roughly an order of magnitude lighter than
its characteristic scale and the difference between the Higgs mass and the mass of
possible new resonances could be explained in a similar way. As will be explained
there, these theories are generically in conflict with bounds from flavor observables,
because they allow for flavor changing neutral currents (FCNCs) at tree level. It will
be shown that, if fermions are partially composite fields, one can avoid most of these
bounds, and also finds an explanation for the hierarchies in the quark sector. Quark
masses and mixings are also unmotivated in the SM, and models which can explain
them are the subject of the second part of the first chapter.