CP-odd Higgs boson production in association with Neutral gauge boson in High-Energy e^+e^- Collisions

We study the associated production of a CP-odd Higgs boson $A^0$ with a neutral gauge boson ($Z$ or photon) in high-energy $e^+ e^-$ collisions at the one-loop level in the framework of Two Higgs Doublet Models (THDM). We find that in the small $\tan\beta$ regime the top quark loop contribution is enhanced leading to significant cross-sections (about a few fb), while in the large $\tan\beta$ regime the cross-section does not attain observable rates.