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Abstract
摘要 |
The composite Higgs models based on the top seesaw mechanism commonly possess an enhanced approximate chiral symmetry, which is spontaneously broken to produce the Higgs field as the pseudo_Nambu-Goldstone bosons (pNGBs). In the minimal model with only one extra vector-like singlet quark that mixes with the top quark, the Higgs arises from the broken U(3)_L chiral symmetry associated with the top, bottom, and the new quark. The mass of the lightest CP-even neutral scalar is strongly correlated with the top quark mass and can naturally be 126 GeV. The minimal model does not have a custodial SU(2) symmetry. As a result, the weak-isospin violation constraint pushes the chiral symmetry breaking scale above a few TeV, which causes a substantial fine-tuning for the weak scale, and also renders the new states beyond the reachof the LHC. A custodial symmetry can be incorporated by extending the model to include a vector-like electroweak doublet of quarks with hypercharge +7/6. In this case the chiral symmetry breaking scale can be lowered to around 1 TeV and the Higgs is a pNGB of the broken O(5) symmetry. For a 126 GeV Higgs boson, the hypercharge +7/6 quarks needs be around or below the chiral symmetry breaking scale, and are likely to be the lightest new states. The 14 TeV LHC will significantly extend the search reach of these quarks. |
