The Limits of Custodial Symmetry
Abstract
We introduce a toy model implementing the proposal of using a custodial symmetry to protect the Zbb coupling from large corrections. This "doublet-extended standard model" adds a weak doublet of fermions (including a heavy partner of the top quark) to the particle content of the standard model in order to implement an O(4) x U(1)X = SU(2)L x SU(2)R x PLR x U(1)X symmetry that protects the Zbb coupling. This symmetry is softly broken to the gauged SU(2)L x U(1)Y electroweak symmetry by a Dirac mass M for the new doublet; adjusting the value of M allows us to explore the range of possibilities between the O(4)-symmetric (M to 0) and standard-model-like (M to infinity) limits.
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