Muon $g-2$ in a $U(1)$-symmetric Two-Higgs-Doublet Model

TL;DR

This paper investigates a $U(1)$-symmetric two-Higgs-doublet model and finds it cannot explain the muon $g-2$ anomaly through neutral Higgs contributions, but a light charged Higgs with large top-Yukawa coupling can account for the anomaly via two-loop diagrams.

Contribution

It demonstrates the near degeneracy of neutral Higgs bosons in the model and identifies conditions under which the muon $g-2$ anomaly can be explained.

Findings

Neutral Higgs bosons are nearly degenerate at large $ aneta$.

The model cannot explain muon $g-2$ via neutral scalars due to mass degeneracy.

A light charged Higgs with large top-Yukawa coupling can account for muon $g-2$ at 1$\sigma$.

Abstract

We show in this paper that, in a $U(1)$-symmetric two-Higgs-doublet model (2HDM), the two additional neutral Higgs bosons would become nearly degenerate in the large $\tan\beta$ regime, under the combined constraints from both theoretical arguments and experimental measurements. As a consequence, the excess observed in the anomalous magnetic moment of the muon could not be addressed in the considered framework, following the usual argument where these two neutral scalars are required to manifest a large mass hierarchy. On the other hand, we find that, with an $\mathcal{O}(1)$ top-Yukawa coupling and a relatively light charged Higgs boson, large contributions from the two-loop Barr-Zee type diagrams can account for the muon $g-2$ anomaly at the 1$\sigma$ level, in spite of a large cancellation between the scalar and pseudoscalar contributions. Furthermore, the same scenario can survive the tight constraints from the $B$-physics observables.