Dipole Showers and Automated NLO Matching in Herwig++

TL;DR

This work introduces a Catani-Seymour based dipole shower implemented as a Herwig++ add-on and an automated framework (Matchbox) for NLO QCD calculations and their matching to the shower. It enables MC@NLO- and POWHEG-like matching via automated dipole subtraction and matrix-element corrections, with ExSample-assisted sampling. The authors validate the approach through LEP, HERA, and Tevatron comparisons, demonstrating good overall agreement and a modest improvement from NLO matching. The study provides a proof-of-concept for automated, framework-wide NLO matching in a modern shower program, paving the way for broader external-code integration and refined treatments of matching systematics.

Abstract

We report on the implementation of a coherent dipole shower algorithm along with an automated implementation for dipole subtraction and for performing POWHEG- and MC@NLO-type matching to next-to-leading order (NLO) calculations. Both programs are implemented as add-on modules to the event generator Herwig++. A preliminary tune of parameters to data acquired at LEP, HERA and Drell-Yan pair production at the Tevatron has been performed, and we find an overall very good description which is slightly improved by the NLO matching.

Figures (11)

• Figure 1: Examples of parton emission from dipole chains. In these examples always the upper dipole has been considered for emissions. Note that any dipole may split in two different ways, splitting either of its legs. These competing possibilities are not shown in the transition diagrams.
• Figure 2: A sketch of the interaction of the Matchbox and dipole shower modules as integrated in Herwig++. To perform a matched NLO calculation an external code only has to provide tree-level and one-loop amplitudes along with colour- and spin-correlated amplitudes of the Born process and an appropriate phase space generator.
• Figure 3: Some event shape variables as predicted by the leading order and next-to-leading order simulations. Here, we additionally compare to the standard Herwig++ shower (version 2.5.1 with default settings), showing that the dipole shower gives a significantly improved description already at leading order.
• Figure 4: The differential three jet rate as predicted by the leading order and next-to-leading order simulations.
• Figure 5: Energy-energy correlation. Note that this observable has not been included in the fit.