Flavor Constraints in a Generational Three Higgs Doublet Model

Abstract: We propose a Three Higgs Doublet Model (3HDM) that goes beyond natural flavor conservation and in which each of the three Higgs doublets couples mainly to a single generation of fermions via non-standard Yukawa structures. A hierarchy in the vacuum expectation values of the three Higgs doublets can partially address the SM flavor puzzle. In light of the experimentally observed $125$ GeV Higgs boson, we primarily work within a 3HDM alignment limit such that a Standard Model-like Higgs is recovered. In order to reproduce the observed CKM mixing among quarks, the neutral Higgs bosons of the theory necessarily mediate flavor changing neutral currents at the tree level. We consider constraints from neutral kaon, $B$ meson, and $D$ meson mixing as well as from the rare leptonic decays $B_s/B^0/K_L\rightarrow\mu^+\mu^-/e^+e^-$. We identify regions of parameter space in which the new physics Higgs bosons can be as light as a TeV or even lighter.