Herwig++ 2.5 Release Note

TL;DR

Herwig++ 2.5 delivers a major upgrade to the event generator by expanding NLO matrix-element support via POWHEG for diboson production and e+e- annihilation, introducing a colour-reconnection model, and enabling photon-initiated and diffractive processes. It also broadens the physics reach with additional BSM frameworks (ADD, leptoquarks, NMSSM, transplanckian scenarios) and new leading-order matrix elements for diverse collision modes, aided by substantial code restructurings for flexibility and MC@NLO compatibility. The release emphasizes improved phenomenology, including LEP and LHC data compatibility, refined shower and hadronization, and extensive bug fixes and stability improvements across ThePEG integration and PDF handling. Collectively, these enhancements extend Herwig++'s applicability to realistic LHC and future collider studies while laying groundwork for the upcoming 3.0 release and the winding down of the FORTRAN HERWIG version.

Abstract

A new release of the Monte Carlo program Herwig++ (version 2.5) is now available. This version comes with a number of improvements including: new next-to-leading order matrix elements, including weak boson pair production; a colour reconnection model; diffractive processes; additional models of physics beyond the Standard Model and new leading-order matrix elements for hadron--hadron and lepton--lepton collisions as well as photon-initiated processes.

Figures (7)

• Figure 1: Comparison of Herwig++, MCFM and MC@NLO for di-boson production.
• Figure 2: Cross section for $e^+e^-\to b\bar{b}$ and thrust distribution at LEP.
• Figure 3: Example distributions comparing an old tune of Herwig++ without colour reconnections to a new tune including colour reconnections.
• Figure 4: Comparison of Herwig++ 2.4.2 and Herwig++ 2.5 to ATLAS minimum-bias distributions at $\sqrt{s}=0.9~\mathrm{TeV}$ with $N_{\mathrm{ch}} \ge 6$, $p_{\perp} > 500~\mathrm{MeV}$ and $|\eta| < 2.5$. The ATLAS data were read off from plots published in Ref. ATLAS-CONF-2010-031.
• Figure 5: Diagram of a two-photon induced process.