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The composite Higgs models based on the top seesaw mechanism commonly possessan enhanced approximate chiral symmetry, which is spontaneously broken toproduce the Higgs field as the pseudo-Nambu-Goldstone bosons. The minimal modelwith only one extra vector-like singlet quark that mixes with the top quark cannaturally give rise to a 126 GeV Higgs boson. However, without having acustodial symmetry it suffers from the weak-isospin violation constraint, whichpushes the chiral symmetry breaking scale above a few TeV, causing asubstantial fine-tuning for the weak scale. We consider an extension to theminimal model to incorporate the custodial symmetry by adding a vector-likeelectroweak doublet of quarks with hypercharge +7/6. Such a setup also protectsthe $Zb\bar{b}$ coupling which is another challenge for many composite Higgsmodels. With this addition, the chiral symmetry breaking scale can be loweredto around 1 TeV, making the theory much less fine-tuned. The Higgs is apseudo-Nambu-Goldstone boson of the broken O(5) symmetry. For the Higgs mass tobe 126 GeV, the hypercharge +7/6 quarks should be around or below the chiralsymmetry breaking scale, and are likely to be the lightest new states. The 14TeV LHC will significantly extend the search reach of these quarks. To probethe rest of the spectrum, on the other hand, would require a higher-energyfuture collider.

Top Seesaw with a Custodial Symmetry, and the 126 GeV Higgs