Twisted Custodial Symmetry in Two-Higgs-Doublet Models

In the standard model for electroweak interactions, the Higgs sector is known to display a custodial symmetry protecting the mass relation m_W^±²=m_W₃² from large corrections. When considering extensions of the scalar sector, this symmetry has to be introduced by hand in order to pass current electroweak precision tests in a natural way. In this Letter, we implement a generalized custodial symmetry in the two-Higgs-doublet model. Assuming the invariance of the potential under $CP$ transformations, we prove the existence of a new custodial scenario characterized by m_H^±²=m_H⁰² instead of m_H^±²=m_A⁰². Consequently, the pseudoscalar A⁰ may be much lighter than the charged H^±, giving rise to interesting phenomenology.