$(g-2{)}_{\mu }$ in the 2HDM and slightly beyond: An updated view

The recent measurement of the muon $g-2$ anomaly continues to defy a Standard Model explanation but can be accommodated within the framework of two-Higgs doublet models, although the pseudoscalar mass must be fairly light. If one further includes extra fermion content in the form of a generation of vectorlike leptons, the allowed parameter range that explains the anomaly is even further extended, and clashes with $B$-decay constraints may be avoided. We show how the muon magnetic moment anomaly can be fit within these models, under the assumption that the vectorlike leptons do not mix with the muon. We update previous analyses and include all theoretical and experimental constraints, including searches for extra scalars. It is shown that the inclusion of vectorlike fermions allows the lepton-specific and muon-specific models to perform much better in fitting the muon’s $g-2$. However, these fits do require the Yukawa coupling between the Higgs and the vectorlike leptons to be large, causing potential problems with perturbativity and unitarity, and thus, models in which the vectorlike leptons mix with the muon may be preferred.

Figure 1

$\mathrm{\Delta }{a}_{\mu }$ allowed regions in the $(m_A,\tan \beta )$ plane for the type-X (top) and type-II (bottom) 2HDMs. The green, yellow, and gray shaded regions were obtained taking the new $\mathrm{\Delta }{a}_{\mu }$ interval given in Eq. (2) at 1, 2 and $3\sigma$, respectively. For comparison, we also show the lines which delimit the same regions when the old result in Eq. (1) is considered.

Figure 2

BZ contributions to $\mathrm{\Delta }{a}_{\mu }$ involving the new charged and neutral leptons $L_a^{-}$ and $N$, respectively.

Figure 3

$(m_A,\tan \beta )$ allowed regions for the type-X, type-II, and $\mu \mathrm{Spec}$ 2HDMs with VLLs (from top to bottom panel), taking the benchmarks B1 and B2 given in (19). The values $s_L=0.5,s_R=0.4$ are chosen. Above the solid (dashed) horizontal line, ${\lambda }_{\max }\equiv \max \{|{\lambda }_L|,|{\lambda }_R|\}>4\pi$ for ${\mathrm{B}}_1$ (${\mathrm{B}}_2$). See text for more details.