Herwig++ Physics and Manual (0803.0883v3)

Abstract: In this paper we describe Herwig++ version 2.3, a general-purpose Monte Carlo event generator for the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron collisions. A number of important hard scattering processes are available, together with an interface via the Les Houches Accord to specialized matrix element generators for additional processes. The simulation of Beyond the Standard Model (BSM) physics includes a range of models and allows new models to be added by encoding the Feynman rules of the model. The parton-shower approach is used to simulate initial- and final-state QCD radiation, including colour coherence effects, with special emphasis on the correct description of radiation from heavy particles. The underlying event is simulated using an eikonal multiple parton-parton scattering model. The formation of hadrons from the quarks and gluons produced in the parton shower is described using the cluster hadronization model. Hadron decays are simulated using matrix elements, where possible including spin correlations and off-shell effects.

• The paper introduces Herwig++ as a Monte Carlo event generator that simulates high-energy collision processes including QCD and BSM phenomena.
• It employs an angular-ordered parton shower using Sudakov form factors and helicity amplitude methods to accurately reproduce key experimental results.
• The manual outlines advancements in hadronization, QED radiation, and BSM flexibility, offering a foundation for further high-energy physics research.

Overview of Herwig++ as a Monte Carlo Event Generator

Herwig++, as outlined in the referenced manuscript, represents a sophisticated tool for simulating events in high-energy lepton-lepton, lepton-hadron, and hadron-hadron collisions. It is a continuation and enhancement of the Herwig legacy and aims to accurately model Quantum Chromodynamics (QCD) processes and Beyond the Standard Model (BSM) physics. The report outlines version 2.3 of Herwig++, documenting its features, including parton shower models, hadronization, and the interface with various matrix element generators. Notably, Herwig++ includes BSM dissemination capabilities and detailed spin correlation mechanisms, suitable for complex particle physics analyses.

Key Features of Herwig++

1. Parton Shower Evolution: Herwig++ employs an angular-ordered parton shower, which improves upon the previous iteration by incorporating exact kinematics and heavy quark fragmentation effects. The parton shower algorithm utilizes Sudakov form factors that manage soft-gluon interferences with a focus on leading-log and some next-to-leading-log accuracy terms. A notable feature is the handling of massive particle radiation through dead-cone effects, making it robust for modeling processes involving heavy particles like the top quark.
2. Matrix Element Integration: Herwig++ adheres to a modular design allowing native matrix element calculations and interfaces to external libraries via the Les Houches Accord format. This flexibility supports various high-energy processes, enhancing its applicability across collider types. For inclusive processes where spin correlations are preeminent, calculations are performed using helicity amplitude methods to preserve spin correlation effects in decay products—critical for processes such as top quark or tau lepton production and decay.
3. Handling of QED Radiation: The incorporation of QED radiation into decay processes is addressed using the YFS formalism. This method precariously handles soft photon emissions and attendant collinear effects, further augmenting the simulation's accuracy in depicting electromagnetic processes in both particle decays and s-channel resonance conditions.
4. BSM Flexibility: Herwig++ provides a robust framework for BSM model implementations. Within this paradigm, new particles and interactions can be defined through Feynman rules, and their implications can be thoroughly explored. The report expounds on several BSM models supported natively, such as variations of Supersymmetry and Universal Extra Dimensions, with extensibility for future models as discoveries at experimental facilities like the LHC unfold.
5. Hadronization with Cluster Models: Hadronization follows the cluster model, segmenting gluons into quark pairs leading to the formation of pre-confinement stable clusters that decay into observed hadrons. The handling of these clusters is further refined through various decaying schemes and cluster fission processes, leading to a realistic portrayal of non-perturbative QCD phenomena.
6. Monte Carlo Event Generation: The generators within Herwig++ operate on principles of unitarity and coherent summation in QCD scattering. As multi-parton interactions are constituent parts of high-energy collisions, the framework incorporates multiple semi-hard and soft partonic scatters to simulate minimum-bias and underlying events more comprehensively.

Numerical Results and Implications

Herwig++ simulators provide robust outputs aligning closely with experimental inputs from previous hadron collider data. Moreover, it can explore the parameter space of both the standard model and BSM physics without significant computational overhead. This capability facilitates thorough analysis, driving theoretical investigations and guiding experimental strategies.

Future Directions

The report also gestures towards improvements and developments in subsequent versions—these include more differential NLO processes accommodations, elaborate BSM model integrations, and potential for higher precision QED treatments. Moreover, multi-parton interaction models will likely be enhanced to better approximate high-luminosity collision environments expected in future experiments.

Overall, Herwig++ serves as an indispensable tool for the particle physics community, harmonizing complex phenomena into tractable simulations and enabling further understanding of the fundamental nature of matter and interactions. Its versatility, accuracy, and extensibility foster a conducive environment for cutting-edge research at the frontier of collider physics.