Herwig 7.2 Release Note

TL;DR

Herwig 7.2 is a major update to the multi-purpose event generator, introducing significant enhancements to parton showers, color handling, spin correlations, and non-perturbative modeling, along with expanded BSM support and MPI refinements. The release integrates angular-ordered and dipole shower improvements, general color matrix element corrections, and POWHEG-style decays, while enabling UFO-based Lorentz structures and new strangeness production mechanisms. It also expands practical capabilities through FxFx merging, updated PDFs, and extensive minor fixes, documentation, and build optimizations, validated against Rivet/Professor and ATLAS data. The combination of these features yields improved accuracy in both perturbative and non-perturbative regimes and broadens applicability to high-energy collider phenomenology and BSM studies.

Abstract

A new release of the Monte Carlo event generator Herwig (version 7.2) is now available. This version introduces a number of improvements, notably: improvements to the simulation of multiple-parton interactions, including diffractive processes; a new model for baryonic colour re-connection; spin correlations in both the dipole and angular-ordered parton showers; improvements to strangeness production; an improved choice of evolution variable in the angular-ordered parton shower; support for generic Lorentz structures in BSM models.

Figures (5)

• Figure 1: The thrust at the Z-pole compared with data from the DELPHI Abreu:1996na experiment. In the right panel a zoom for small $1-T$ values is shown.
• Figure 2: Examples of the spin correlations in the parton shower for $g\to gg$ with subsequent $g\to gg$ and $g\to q\bar{q}$ branching. For details, see Ref. Richardson:2018pvo.
• Figure 3: The K to $\pi$ ratio in inelastic events in comparison with ALICE dataAdam:2015qaa.
• Figure 4: The charged-particle multiplicity is plotted against the rapidity for multiple cuts (green labels) on the hardest track transverse momentum and number of charged particles. Data is taken from Aad:2010ac. This observable is sensitive to the choices that are employed as the starting conditions of the parton shower process. The four choices are described in Sec. \ref{['sec:mpichanges']}. While height differences are easily modified in the tuning process, shape differences prefer choices with a random colour partner for gluons in the hard process.
• Figure 5: The cross section for Z production in association with $N_{\textrm{jets}}$ jets (upper) or differentially with respect to the total scalar sum of final state transverse momenta, $H_{\textrm{T}}$, (lower) in comparison with ATLAS dataAaboud:2017hbk.