Threshold Effects in Two--Photon Decays of Higgs Particles

The couplings of Higgs particles to two photons are analyzed in the threshold region, where the Higgs mass is about twice the loop--particle mass leading to possible bound state formation. The amplitude of the pseudoscalar decay $A\to \gamma\gamma$ for pseudoscalar masses in the range of $(t\bar t)$--bound state masses is calculated in leading and next--to--leading order in $\alpha_s$. It is shown that the real and imaginary parts of the form factor are significantly different from the lowest order perturbative one.