Current bounds on the Type-Z $Z_3$ three Higgs doublet model

Abstract

Type-Z models, where charged leptons, up type quarks, and down type quarks each couple to a different scalar, are only possible when there are three or more Higgs doublets. We consider the Type-Z three Higgs doublet model imposed by a softly broken $Z_3$ symmetry. We take into account all theoretical and experimental constraints, highlighting the importance of perturbative unitarity and bounded from below conditions that we develop here, and which have a strong impact on $B \rightarrow X_s \gamma$. Since there can be cancellations between the two charged Higgs in $B \rightarrow X_s \gamma$ (and in $h \rightarrow \gamma \gamma$), the lower bounds obtained on the charged Higgs masses are alleviated. We also discuss in detail the important physical differences between exact alignment and approximate alignment, and present some useful benchmark points.