The $\gamma \gamma \to A^0 A^0$ process at a $\gamma \gamma $ Collider
  The helicity amplitudes for the process $\gamma \gamma \to A^0 A^0$ are studied to 1-loop order in the minimal SUSY (MSSM) model, where $A^0$ is the CP-odd Higgs particle.   Simple exact analytic formulae are obtained, in terms of the $C_0$ and $D_0$ Passarino-Veltman functions; in spite of the fact that the loop-diagrams often involve different particles running along their sides. For a usual mSUGRA set of parameters, $\sigma (\gamma \gamma \to A^0 A^0) \sim (0.1-0.2)\rm fb $ is expected. If SUSY is realized in Nature, these expressions should be useful for understanding the Higgs sector. 
G.J. Gounaris and P.I. Porfyriadis