Production of two Z-bosons in gluon fusion in the heavy top quark approximation

We compute QCD radiative corrections to the continuum production of a pair of Z-bosons in the annihilation of two gluons. We only consider the contribution of the top quark loops and we treat them assuming that $m_t$ is much larger than any other kinematic invariant in the problem. We estimate the QCD corrections to $pp \to ZZ$ using the first non-trivial term in the expansion in the inverse top quark mass and we compare them to QCD corrections of the signal process, $pp \to H \to ZZ$.