Large mass expansion in two-loop QCD corrections of para-charmonium decay

We calculate the two-loop QCD corrections to paracharmonium decays $eta_{c} rightarrow gamma gamma$ and $eta_{c} rightarrow g g$ involving light-by-light scattering diagrams with light quark loops. Artificial large mass expansion and convergence improvement techniques are used to evaluate these corrections. The obtained corrections to the decays $eta_{c} rightarrow gamma gamma$ and $eta_{c} rightarrow g g$ account for $-1.25$ and $-0.73$ of the leading order contribution, respectively.