Heavy-quark box-loop corrections to $q\bar q \to Z\gamma$ at two loops in QCD

Abstract

We numerically compute the two-loop QCD corrections to $Z\gamma$ production at the LHC mediated by light- and heavy-quark box loops. The calculation employs the pipeline of refs. arXiv:2407.18051 and arXiv:2510.18801, which performs Monte Carlo integration over spatial loop momenta after local subtraction of infrared, ultraviolet, and threshold singularities. We validate our results for partonic squared matrix elements with massless-quark loops against known benchmarks, extend them to include heavy-quark contributions, and compute the double-virtual corrections to $pp\to Z\gamma$ by performing the loop and phase space integrations simultaneously. This computation demonstrates the flexibility of the approach in handling both massless and massive final-state bosons, as well as additional mass scales in the loop.