A ``littlest Higgs'' model with custodial SU(2) symmetry

In this note, a ``littlest higgs'' model is presented which has anapproximate custodial SU(2) symmetry. The model is based on the coset space$SO(9)/(SO(5)\times SO(4))$. The light pseudo-goldstone bosons of the theoryinclude a {\it single} higgs doublet below a TeV and a set of three $SU(2)_W$triplets and an electroweak singlet in the TeV range. All of these scalarsobtain approximately custodial SU(2) preserving vacuum expectation values. Thismodel addresses a defect in the earlier $SO(5)\times SU(2)\times U(1)$ moosemodel, with the only extra complication being an extended top sector. Some ofthe precision electroweak observables are computed and do not deviateappreciably from Standard Model predictions. In an S-T oblique analysis, thedominant non-Standard Model contributions are the extended top sector and higgsdoublet contributions. In conclusion, a wide range of higgs masses is allowedin a large region of parameter space consistent with naturalness, where largehiggs masses requires some mild custodial SU(2) violation from the extended topsector.Comment: 22 pages + 8 figures; JHEP style, added references and extra discussion on size of T contributions, as well as some other minor clarifications. Version to appear in JHE