Multi-Higgs doublet models: the Higgs-fermion couplings and their sum rules

Understand

This is the second of a series of papers that explores the physical parameterization, sum rules and unitarity bounds arising from a non-minimal scalar sector of the Standard Model (SM) that consists of N Higgs doublets.

• In this paper, we focus on the structure and implication of the Yukawa interactions that couple the N scalar doublets to the SM fermions.
• We employ the charged Higgs basis, which is defined as the basis of scalar fields such that the neutral scalar field vacuum expectation value resides entirely in one of the N scalar doublet fields, and the charged components of the remaining N-1 scalar doublet fields are the physical (mass-eigenstate) charged Higgs fields.
• Based on the structure of the Yukawa Lagrangian of the model (and as a consequence of tree-level unitarity), one may deduce numerous sum rules, several of which have not appeared previously in the literature.