Next-to-next-to-leading order event generation for $t\bar{t}H$ production with approximate two-loop amplitude

TL;DR

This paper introduces the first NNLO QCD event generator for $t\bar{t}H$ production using the MiNNLOPS method, employing approximate two-loop amplitudes and validating the approach with phenomenological results.

Contribution

It presents a novel NNLO event generator for $t\bar{t}H$ production with approximate two-loop amplitudes, combining soft Higgs and high-energy limits, and provides publicly available tools.

Findings

NNLO corrections significantly impact $t\bar{t}H$ predictions.

The combined approximation method yields reliable differential results.

Validation confirms the robustness of the new NNLO event generator.

Abstract

We study Higgs-boson production in association with a top-quark pair ($t\bar{t}H$) at hadron colliders and present the first matching of next-to-next-to-leading order (NNLO) QCD corrections to parton showers using the MiNNLOPS method. For the two-loop amplitude, we employ two established approximations, based on the soft Higgs-boson and high-energy limits, respectively. For the first time, we also construct the latter in full colour and propose a pointwise combination of the two approximations across phase space. By assigning a conservative uncertainty estimate, which remains well below the perturbative uncertainties, we ensure robust and reliable differential predictions, explicitly validated at the one-loop level. Apart from the two-loop amplitude, all remaining ingredients of the MiNNLOPS calculation are included exactly. After thorough validation, we present a series of phenomenological results illustrating the impact of NNLO corrections and parton-shower effects. We consider fiducial predictions for the Higgs-boson decay into photons and include off-shell top-quark decays with tree-level spin correlations in both the dilepton and semileptonic channels. Our $t\bar{t}H$ MiNNLOPS generator is publicly available within the POWHEG framework.