A custodial symmetry for Zbb¯

Abstract

We show that a subgroup of the custodial symmetry O(3) that protects �ρ from radiative corrections can also protect the Zbcoupling. This allows one to build models of electroweak symmetry breaking, such as Higgsless, Little Higgs or 5D composite Higgs models, that are safe from corrections to Z → bb. We show that when this symmetry protects Zbit cannot simultaneously protect Ztand W t ¯. Therefore one can expect to measure sizable deviations from the SM predictions of these couplings at future collider experiments. We also show under what circumstances ZbRR can receive corrections in the right direction to explain the anomaly in the LEP/SLD forward-backward asymmetry A b B.