Top quark mass effects in $gg\to ZZ$ at two loops and off-shell Higgs interference

We consider top-quark mass effects in the Higgs-interference contribution to $Z$-boson pair production in gluon fusion. While this production mechanism is formally of next-to-next-to leading order, its contribution is numerically important above the top threshold $M_{ZZ}^2=4m_t^2$. This region is essential to constrain the width of the Higgs boson and good control over the top-quark mass dependence is crucial. We determine the form factors that are relevant for the interference contribution at two-loop order using a method based on a conformal mapping and Pad\'e approximants constructed from the expansions of the amplitude for large top mass and around the top threshold.