ABSTRACT
This chapter provides a partial survey of the literature in the area, with some description of the underlying physics and of the kinds of predictions that help distinguish these mechanisms from others. The phase leads to CP-violating couplings of fermions to charged Higgs bosons, analogous to the charged current couplings of the Standard Model. The original axion model was just an ordinary two-Higgs doublet model in which the pseudoscalar was massless at tree level. The only way that one can avoid the severe flavor-changing neutral currents constraints on the Higgs masses is to not use the bi-doublet structure to generate quark masses. The field can couple to the left-handed neutrino, so that the neutrino acquires a Majorona mass when the triplet Higgs field develops a vacuum expectation value. The decays of the Higgs bosons are primarily determined by the allowed Yukawa couplings in the model.
