Fermion number conservation and two-Higgs-doublet models without tree-level flavor-changing neutral currents

Abstract

The charged fermion mass matrices are always invariant under $U(1{)}^{3}$ symmetry linked to the fermion number transformation. A class of two-Higgs-doublet models can be identified by requiring that the definition of this symmetry in an arbitrary weak basis be independent of Higgs parameters such as the ratio of the Higgs vacuum expectation values. The tree-level flavor changing neutral currents normally present in two-Higgs-doublet models are absent in this class of models, but unlike the type-I or type-II Higgs doublet models, the charged Higgs couplings in these models contain additional flavor-dependent CP-violating phases. These phases can account for the recent hints of the beyond-standard-model CP violation in the ${B}_{d}$ and ${B}_{s}$ mixing. In particular, there is a range of parameters in which new phases do not contribute to the $K$meson CP violation but give identical new physics contribution to the ${B}_{d}$ and ${B}_{s}$meson mixing. Specific model realizations of the above scenario are briefly discussed.