Improving Parton Shower Physics in Herwig 7
The next era of particle physics is rapidly approaching. With the upcoming High-Luminosity LHC and proposals for a Future Circular Collider, experiments will deliver collision data with unprecedented precision. Our simulation tools must match that accuracy.
While significant progress has been made in fixed-order calculations, the event-generator and resummation communities are now pushing parton showers to higher logarithmic accuracy. As of 2026, several formalisms achieve Next-to-Leading-Log (NLL) accuracy — including:
- PanLocal and PanGlobal
https://gsalam.web.cern.ch/panscales/ - FHP (Forshaw–Holguin–Plätzer)
https://arxiv.org/abs/2605.02622 - Alaric
https://arxiv.org/abs/2404.14360 - Apollo
https://arxiv.org/abs/2403.19452
Exploratory steps toward NNLL precision are now underway.
This project integrates the PanGlobal and FHP showers into Herwig, alongside its existing Catani–Seymour shower, enabling NLL-accurate predictions within mainstream collider simulations.
We discovered spurious low-$p_T$ effects, where these showers generated very hard, very collinear emissions, that alter the predicted particle spectra, a systematic error the field had overlooked. To understand the impact of this effect, we introduced a dynamic shower cutoff that bounds the generation to the range of the older showers.
