Herwig++ 2.0 Release Note

TL;DR

Herwig++ 2.0 provides a comprehensive hadron-hadron event generator by implementing full initial- and final-state QCD radiation, expanded matrix-element elements, and ME corrections. The release integrates a coherent heavy-particle decay shower, an UA5-based underlying-event model, QED radiation via YFS formalism, and substantial code improvements including a Shower redesign and new decayers. It also delivers extensive bug fixes, new input configurations, and infrastructure updates to pave the way for multi-scale showers and future BSM extensions in version 3.0. Overall, the paper details a first complete Hadron-Hadron physics capability in Herwig++ and outlines planned enhancements that will extend its applicability to LHC-era simulations and beyond.

Abstract

A new release of the Monte Carlo program Herwig++ (version 2.0) is now available. This is the first version of the program which can be used for hadron-hadron physics and includes the full simulation of both initial- and final-state QCD radiation.

Figures (4)

• Figure 1: The transverse momentum of the $Z^0$ boson at the Tevatron calculated by HERWIG6.5 and Herwig++ compared with run I CDF data taken from Affolder:1999jh with and without matrix element corrections.
• Figure 2: Dalitz plot for gluonic radiation in top decay. In both plots both the soft and hard matrix element corrections have been applied, but only one emission has been allowed. a) shows the radiation for the symmetric choice of Gieseke:2003rz for emission from the top and bottom while b) shows the radiation with the scales chosen to give the maximum amount of radiation from the bottom quark. The blue (innermost) line gives the limit for radiation from the bottom, the green (middle) line from the top and the red (outer) line the boundary of the phase-space region.
• Figure 3: The number of charged particles and scalar sum of the transverse momentum in the transverse azimuthal direction with respect to the direction of the leading jet at the Tevatron as a function of the transverse momentum of the leading jet. The blue (upper) and black (lower) points are the results from Herwig++ with/without the soft underlying event. The light green (upper) and dark green (lower) points were generated with HERWIG with/without the soft underlying event. For both the number of charged particles and transverse the FORTRAN results lie slightly above those from Herwig++. The observables are defined in more detail in Affolder:2001xt.
• Figure 4: The total energy of the photons radiated in ${\rm W}^\pm\to{\rm e}^\pm\nu_e/\bar{\nu}_e$ decays. In figure (a) the red (dashed) histogram was generated using the WINHACPlaczek:2003zg program, while the black (solid) line was generated using Herwig++. Figure (b) shows the difference between the spectra shown in (a) divided by the statistical error. The disagreement about 40 GeV is due to events with at least two hard photons which neither program is designed to model well.