WHIZARD: Simulating Multi-Particle Processes at LHC and ILC (0708.4233v2)

Abstract: We describe the universal Monte-Carlo event generator WHIZARD. The program automatically computes complete tree-level matrix elements, integrates them over phase space, evaluates distributions of observables, and generates unweighted event samples that can be used directly in detector simulation. There is no principal limit on the process complexity; using current hardware, the program has successfully been applied to hard scattering processes with up to eight particles in the final state. Matrix elements are computed as helicity amplitudes, so spin and color correlations are retained. The Standard Model, the MSSM, and many alternative models such as Little Higgs, anomalous couplings, or effects of extra dimensions or noncommutative SM extensions have been implemented. Using standard interfaces to PDF, beamstrahlung, parton shower and hadronization programs, WHIZARD generates complete physical events and covers physics at hadron, lepton, and photon colliders.

• The paper demonstrates that WHIZARD accurately simulates multi-particle interactions by computing complete tree-level matrix elements and handling up to eight final-state particles.
• It employs modular integration and adaptive multi-channel techniques to optimize event weight distributions and manage sharp resonances.
• The tool supports diverse models such as MSSM and Little Higgs and features SINDARIN scripting for flexible collider simulation configurations.

WHIZARD: Simulating Multi-Particle Processes at LHC and ILC

The paper provides a comprehensive exploration of the WHIZARD software, a universal Monte-Carlo event generator tailored for simulating multi-particle processes at the Large Hadron Collider (LHC) and the future International Linear Collider (ILC). WHIZARD is manifested as a modular tool that encompasses the selection, parameterization, and integration of complex particle interactions inherent in high-energy physics experimental setups.

Originating from the need to simulate processes far beyond the simple scattering events historically handled by Monte-Carlo tools like PYTHIA and HERWIG, WHIZARD addresses the challenges posed by intricate weak-boson fusion and scattering processes, $t\bar tH$ production, supersymmetric (SUSY) cascades, and other non-Standard Model (SM) phenomena. Beyond these complexities, the software successfully integrates processes involving up to eight final-state particles. This advancement is primarily enabled by WHIZARD’s ability to compute complete tree-level matrix elements and organize them into comprehensive event samples.

Notably, WHIZARD differentiates itself with capabilities that span multiple theoretical frameworks, including the MSSM, Little Higgs models, and scenarios with anomalous couplings or extra-dimensional effects. This versatility is achieved through its design, which allows for process concatenation, maintaining full spin correlations—facilitating factorized approximations without compromising on matrix element completeness.

Numerical Results and Computational Strategies

WHIZARD's utility is underscored by its numerical robustness. It efficiently handles QCD and electroweak processes with significant particle multiplicities, presenting an integration framework that adapts multi-channel techniques. The VAMP library, a part of WHIZARD, optimizes event weight distributions during Monte-Carlo integration by dynamically adjusting sampling strategies to handle sharp resonances and phase space intricacies.

Tables of cross-sections, derived from WHIZARD's exhaustive simulations, affirm its precision. Processes such as $W + njets$ (where n up to 5 has been considered) and top pair production—including decay chains—stand as testament to its scalability and accuracy. This capability to maintain numerical stability over extensive variables and computational grids is crucial, especially under the demanding conditions of real collider environments.

Extensions and Future Directions

The paper also remarks on WHIZARD's extensibility through its scripting language, \SINDARIN/, which offers a comprehensive syntax for configuring physics parameters, cut definitions, and specifying event samples. The use of external libraries like LHAPDF for structure functions and the provision for interfacing with parton shower programs via the Les Houches Accord events format further enhance its extensibility.

Looking ahead, the prospect of incorporating higher-order corrections remains a promising frontier for WHIZARD. While currently limited to tree-level processes, the authors discuss pathways to accommodate NLO corrections and develop consistent parton shower matching methods. Such enhancements are crucial for aligning with the granularity of experimental data expected from both LHC and future colliders.

WHIZARD emerges as a formidable tool in the computational physics landscape, adeptly balancing the detailed requirements of high-energy physics simulations with the need for a broad, flexible framework accommodating a myriad of theoretical models. As collider physics endeavors advance, WHIZARD's development trajectory holds the potential to significantly influence the precision and reliability of next-generation simulations.