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. The amplitude of the pseudoscalar decay A→γγ for pseudoscalar masses in the range of (tt-bar)-bound state masses is calculated in leading and next-to-leading order in αs. It is shown that the real and imaginary parts of the form factor are significantly different from the lowest order perturbative one. 27 refs.; 5 figs