A Resolution of the Flavor Problem of Two Higgs Doublet Models with an Extra U(1)_H Symmetry for Higgs Flavor

TL;DR

This paper introduces a new gauge symmetry U(1)_H in two Higgs doublet models to naturally prevent flavor-changing neutral currents, providing a gauge-based alternative to ad hoc symmetry assumptions.

Contribution

It proposes implementing an extra U(1)_H gauge symmetry with different charges for Higgs doublets to avoid tree-level FCNCs, replacing the need for Z_2 symmetry and analyzing anomaly cancellation.

Findings

U(1)_H can be leptophobic or leptophilic in different 2HDM types

U(1)_H models include U(1)_{B-L}, U(1)_R, or U(1)_Y as special cases

The models have distinctive phenomenological features

Abstract

We propose to implement the Natural Flavor Conservation criterion in the two Higgs doublet model (2HDM) to an extra U(1)_H gauge symmetry for Higgs flavor, assuming two Higgs doublets carry different U(1)_H charges. Then one can easily avoid the tree level FCNC from neutral Higgs mediations using local gauge symmetries, instead of softly broken ad hoc Z_2 symmetry, and the pseudoscalar boson could be eaten by extra Z_H boson. Imposing the anomaly cancellation, we find that the U(1)_H in the Type-II and Type-IV 2HDM's become leptophobic and leptophilic, respectively. For the Type-I case, U(1)_H depends on two parameters, and some simple cases include U(1)_H = U(1)_{B-L}, U(1)_R, or U(1)_Y. We sketch qualitative phenomenology of these models.