NLO QCD corrections in Herwig++ with MC@NLO

TL;DR

This work delivers the full MC@NLO-compatible MC subtraction terms for Herwig++, enabling NLO QCD accuracy in an updated, widely-used event generator. It implements the formalism for all processes previously interfaced to Fortran HERWIG and provides comparisons showing broad agreement with the legacy setup, while revealing differences driven by the newer shower model, especially in heavy-quark production. The results validate the approach and demonstrate the practical impact on Higgs, tt̄, and single-top predictions, including realistic cut effects. Looking forward, the paper argues for leveraging automatic NLO matrix-element generation to extend MC@NLO+Herwig++ to a much wider class of processes, strengthening precision predictions for LHC phenomenology.

Abstract

We present the calculations necessary to obtain next-to-leading order QCD precision with the Herwig++ event generator using the MC@NLO approach, and implement them for all the processes that were previously available from Fortran HERWIG with MC@NLO. We show a range of results comparing the two implementations. With these calculations and recent developments in the automatic generation of NLO matrix elements, it will be possible to obtain NLO precision with Herwig++ for a much wider range of processes

Figures (14)

• Figure 1: MC@NLO results on Higgs boson production: Higgs transverse momentum (left) and rapidity (right) distributions with Herwig++ (solid) and HERWIG6 (dashed).
• Figure 2: MC@NLO results on Higgs $\to WW\to l\nu l\nu$ for Herwig++ (solid) and for HERWIG6 with (dashed) and without (dotted) spin correlations. Left panel: azimuthal angle between the charged leptons. Right panel: dilepton invariant mass.
• Figure 3: MC@NLO results on top quark pair production: $\log_{10}$ of the $t\bar{t}$ transverse momentum distribution with Herwig++ (solid) and HERWIG6 (dashed). Left/right panel: without/with acceptance cuts.
• Figure 4: MC@NLO results on top quark pair production: $t\bar{t}$ azimuthal separation with Herwig++ (solid) and HERWIG6 (dashed). Left/right panel: without/with acceptance cuts.
• Figure 5: MC@NLO results on top quark pair production: $b$-quark transverse momentum distribution with Herwig++ (solid) and HERWIG6 (dashed). Left/right panel: without/with acceptance cuts.