THE LIMITS OF CUSTODIAL SYMMETRY

We introduce a toy model implementing the proposal of using a custodial symmetry to protect the Zb(L)(b) over bar (L) 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) similar to SU(2)(L) x SU(2)(R) x P-LR x U(1)(X) symmetry in the top-quark mass generating sector. 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 -> 0) and standard-model-like (M -> infinity) limits.