E⁺e^- → vv̄A⁰ in the two-Higgs-doublet model

We compute the cross section for e⁺e^- → vv̄A⁰ in the general CP-conserving type-II two-Higgs-doublet model. We sum the contributions from the "t-channel" e ⁺e^- → vv̄WW → vv̄A⁰ graphs and "s-channel" e⁺e^- → ZA⁰ → vv̄A⁰ graphs, including their interference. Higgs-triangle graphs and all box diagrams are included. For many parameter choices, especially those in the decoupling region of parameter space (light h⁰ and m_A0,m_H0,m_H± > 2m_Z), the Higgs-triangle and box diagrams are found to be of minor importance, the main contributing loops being the top and bottom quark triangle diagrams. The predicted cross section is rather small for tan β>2 and/or m_A0 > 2m_t. However, we also show that if parameters are chosen corresponding to large Higgs self-couplings then the Higgs-triangle graphs can greatly enhance the cross section. We also demonstrate that the supersymmetry-loop corrections to the bb̄A⁰ coupling could be such as to greatly enhance this coupling, resulting in an enhanced vv̄A⁰ cross section. Complete cross-section expressions are given in the Appendixes.