General-purpose event generators for LHC physics (1101.2599v1)

Abstract: We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hard-scattering matrix elements for processes of interest, at both leading and next-to-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diffractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the ARIADNE, Herwig++, PYTHIA 8 and SHERPA generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.

• The paper demonstrates the integration of leading and next-to-leading order QCD matrix elements with parton showers to accurately model proton-proton collisions.
• It evaluates modular frameworks like Pythia, Herwig, and Sherpa, highlighting unique approaches to hadronization and multi-parton interactions.
• The research emphasizes tuning generators with experimental data to refine Standard Model predictions and explore potential new physics signals.

Overview of the Paper: "General-purpose event generators for LHC physics"

The reviewed paper provides a foray into the intricacies of Monte Carlo event generators, pivotal tools in simulating proton-proton collisions at the Large Hadron Collider (LHC). These generators serve as essential resources for transforming theoretical predictions into simulated hadron-level events, crucial for interpreting data from high-energy physics experiments. The paper articulates the physics foundations, principal characteristics, and applications of these generators while addressing their contribution to the bridge between parton-level and hadron-level theories.

Technical Summary

The paper delineates the multifaceted components of general-purpose Monte Carlo event generators, encompassing the generation of hard-scattering matrix elements at both leading (LO) and next-to-leading order (NLO) in Quantum Chromodynamics (QCD), integration with parton showers, and the implementation of parton distribution functions (PDFs). It meticulously examines the physical processes modeled, from perturbative interactions to non-perturbative phenomena such as soft QCD collisions, hadronization, and QED radiation.

A detailed discussion is provided on the modular frameworks of event generators like Ariadne, Herwig, Pythia, and Sherpa, each with unique algorithms for generating final states in hadron collisions. The generators employ sophisticated techniques such as dipole-based parton showers and cluster or string models for hadronization, effectively modeling the transition from colored partons to color-neutral hadrons. The paper also elaborates on the matching algorithms, such as CKKW and POWHEG, designed to combine matrix elements with parton showers, ensuring a seamless and accurate representation of multi-parton events.

Special focus is granted to the complexities of soft QCD and the underlying event, where multiple parton interactions (MPI) and color coherence effects play a critical role. The challenge of simulating diffractive events using models based on pomerons is also addressed.

Numerical Results and Claims

Throughout the paper, numerous examples underscore the efficacy of these event generators in reproducing experimental results. The authors present strong numerical alignment between generator predictions and data from LHC experiments, highlighting the role of tuning against experimental observables. The development and validation of these generators are positioned as critical for deciphering both Standard Model predictions and potential signals of new physics.

Implications and Future Outlook

The implications of this research are multi-fold. Practically, these generators form the backbone of experimental analysis at the LHC, aiding in the precise extraction of QCD parameters and the identification of novel phenomena. Theoretically, they provide a rigorous testing ground for QCD and its extensions, pushing the boundaries of perturbative calculations and experimental validation.

Speculating on future advancements, the paper suggests enhancements in the automation of NLO and possibly NNLO calculations within these generators, improving their accuracy and computational efficiency. The continuous refinement of hadronization models and the potential incorporation of machine learning techniques stand as promising avenues for future research in Monte Carlo event simulation.

In conclusion, the synthesis of this research underscores the indispensable nature of event generators in contemporary high-energy physics, highlighting their sophisticated design and adaptability to evolving experimental paradigms. The paper marks a seminal contribution to the ongoing development of high-fidelity simulations that are paramount for the success of collider physics endeavors.