“Flavorful” naturalness from light quark compositeness
Theorists at KAIST and CERN have built a model to help study the compositeness of light generation quarks in composite Higgs models with the right-handed up-type quarks having a large degree of compositeness, and conducted a joint analysis with the data from recent experiments at the Large Hadron Collider (LHC). It has been shown that light non-degenerate composite partners allow for a flavorful naturalness solution for so called fine-tuning problem associated with recently discovered Higgs boson (why the scalar mass is not affected by quantum corrections which would be 17 orders of magnitude larger than the Higgs mass itself!).
With the recent discovery of the Higgs boson at the now famous Large Hadron Collider (LHC), identifying the origin of electroweak symmetry breaking is one of the most important tasks for particle physics to undertake. One of a few motivated models addressing the so-called fine-tuning problem is the framework of composite Higgs models with the Higgs as a pseudo-Nambu-Goldstone-boson associated with the spontaneous breakdown of an approximate global symmetry. In this class of model, the Higgs potential is radiatively generated by resonance loops, with the largest contribution coming from the top sector. In order to be consistent with the measured value of the Higgs mass around 125 GeV, top quark partner mass is required to be light, bellow 1 TeV. However, the searches from the LHC pushes the exclusion limit for the light top partner to be 800 GeV or above. In deriving such limit for the top partner mass, there is an assumption that top partners are pure mass eigenstates. However, the top partner flavor eigenstate can consist of an admixture of would be top-partner-like and charm-partner-like mass eigenstate, resulting in ”flavorful” naturalness scenario with a weaker bound on the top partner mass and potentially improving on the EW scale fine-tuning. Theorists at KAIST and CERN have investigated this idea in the context of the composite Higgs model, and explore the phenomenology of light generation quark compositeness, addressing the question of how low in weight can the first and second generation quark partners be without assuming degenerate compositeness parameters.
Various relevant ATLAS and CMS analyses have been reinterpreted in order to constrain the parameter space of this class of models. In the limit of first two generation degeneracy, as in minimal flavor violation or U(2)-symmetric flavor models, fourplet partners need to be relatively heavy, with masses above 1.8 TeV, or the level of compositeness needs to be rather small. The situation is significantly different in models which deviate from the first two generation degeneracy paradigm, as charm quark parton distribution functions are suppressed relative to the up quark ones. It is found that the right-handed charm quark component can be mostly composite together with their partners being as light as 600 GeV, while the right-handed up quark needs either to be mostly elementary, or to have partners as heavy as 2 TeV. Models where right-handed up type quarks are fully composite fermions are also analyzed and yield qualitatively similar conclusions. Therefore, ”flavorful” naturalness scenario is shown to be a viable theory option to address the fine-tuning problem.
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