  In this study, the direct pair production of the pseudoscalar Higgs boson at
a photon-photon collision is analyzed in the context of two Higgs doublet
model, taking account the complete one-loop contributions. In order to
illustrate the effect of the new physics, four benchmark points scenarios,
which are consistent with theoretical and current experimental constraints, are
chosen in the type-I of THDM with an exact alignment limit. In these benchmark
points, the CP even lightest Higgs boson ($h^0$) have the Standard Model like
couplings to the gauge bosons. The effect of individual contributions from each
type of one-loop diagrams on the total cross section are examined in detail.
The dependence of total cross section on the center-of-mass energy is also
presented at the various polarization configurations of the incoming photons.
Moreover, the regions $m_{12}^2-\tan\beta$ and $m_A-\tan\beta$ in the parameter
space of the THDM are scanned for some fixed values of other parameters. The
box-type diagrams make a much larger contribution to the total cross section
than the others at high energies. Total cross section can be enhanced by a two
factor thanks to opposite polarized photons as well as threshold effects.

High Energy Physics - Phenomenology