NNLO QCD corrections to $γγ\rightarrow Q\bar{Q}$ from Local Unitarity combined with Coulomb resummation and NLO EW effects
Zeno Capatti, Mathijs Fraaije, Valentin Hirschi, Lucien Huber, Ben Ruijl, Hua-Sheng Shao
Published: 2025/9/16
Abstract
The Local Unitarity formalism provides a constructive, integrand-level realisation of the Kinoshita-Lee-Nauenberg theorem, by combining loop and phase-space integrals appearing in scattering cross-sections in such a way that their final-state infrared singularities cancel before integration. Supplemented with localised ultraviolet renormalisation, it enables the direct Monte Carlo integration of (differential) cross sections at arbitrary perturbative order in four-dimensional spacetime. In this paper, we present its application to the Next-to-Next-to-Leading Order (NNLO) QCD cross sections for heavy-quark pair production in direct photon fusion, involving the contribution from 138 distinct forward-scattering diagrams where external photons couple only to heavy quarks. By combining NNLO QCD with Next-to-Leading Order (NLO) ElectroWeak (EW) corrections and Next-to-Leading Power (NLP) Coulomb resummation, we obtain state-of-the-art predictions for top-, bottom-, and charm-quark production in ultraperipheral hadron collisions and at $e^+ e^-$ colliders.