Muon g−2 in the 2HDM: Maximum results and detailed phenomenology

We analyze the maximum contributions to the muon magnetic moment $a_{\mu }$ and Yukawa and triple Higgs couplings in the flavor-aligned two-Higgs doublet model (2HDM). We focus on the most promising case of a light pseudoscalar Higgs $A$ with large Yukawa couplings to leptons and quarks. Taking into account experimental constraints from LHC Higgs and flavor physics, we find maximum possible Yukawa couplings of a light $A$ of around 50–100 (leptons) and $\mathcal{O}(0.5)$ (quarks). An overall maximum of $a_{\mu }$ of more than $45\times {10}^{-10}$ is possible in a very small parameter region around $M_A=20\mathrm{GeV}$. For $M_A$ up to 100 GeV, the maximum possible value of $a_{\mu }$ is compatible with the currently observed deviation if the $A$ couplings to quarks and leptons are both large, making this scenario promising for LHC searches. We also analyze the subleading bosonic two-loop contributions to $a_{\mu }$, finding values up to $3\times {10}^{-10}$.