Virtual QCD corrections to $gg\to ZZ$: top-quark loops from a transverse-momentum expansion

We present the virtual corrections due to the top-quark loops for the process $gg\to ZZ$ at next-to-leading order in QCD. The associated two-loop box diagrams are computed using a small-transverse-momentum expansion. Our results are then merged with those available in the complementary energy region, obtained via a high-energy expansion, in order to provide an analytic result that is valid in the whole phase space. The results presented allow for an efficient modelling of the signal–background interference as well as the irreducible background in off-shell Higgs production.