# Event generator

An event generator is a [Monte Carlo](https://www.edgepedia.org/monte-carlo) computer program that produces simulated particle-collision events, that is, complete lists of the particles produced in a collision together with their momenta, so that theoretical models of high-energy physics can be compared directly with what a detector would measure. In modern terminology such programs are described as the digital twins of collider-based particle physics experiments<sup>[1](https://arxiv.org/abs/2509.21611)</sup>. They are one of the computational pillars of every high-energy physics experiment, including the LHC collaborations<sup>[2](https://link.springer.com/article/10.1007/s41781-021-00055-1)</sup>.

| Key fact | Detail |
|---|---|
| Purpose | Generate exclusive collision events, from a hard partonic scattering down to hadronization and decays, connecting first-principles calculations to experimental measurements<sup>[3](https://arxiv.org/abs/2406.08026)</sup> |
| Main components | Hard scattering matrix elements, parton showers, underlying event, hadronization, and hadron and tau decays<sup>[4](https://www.sciencedirect.com/science/article/abs/pii/S0370157311000846)</sup> |
| Leading general-purpose generators | Pythia, Herwig, and Sherpa, each with decades of development<sup>[3](https://arxiv.org/abs/2406.08026)</sup> |
| Hadronization models | The Lund string model (default in Pythia) and the cluster model (used in Herwig and Sherpa)<sup>[5](https://link.springer.com/article/10.1140/epjc/s10052-025-15182-x)</sup> |
| NLO matching | MC@NLO and POWHEG combine parton showers with fixed-order perturbation theory for next-to-leading-order accuracy<sup>[6](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-102711-094928)</sup> |
| Standard interfaces | Les Houches Event Files (LHEF) pass parton-level events to general-purpose generators; HepMC stores final-state events<sup>[7](https://arxiv.org/abs/hep-ph/0609017)</sup><sup> • </sup><sup>[8](https://arxiv.org/abs/1912.08005)</sup> |
| Validation and tuning | The Rivet toolkit validates against benchmark analyses; Professor tunes phenomenological free parameters<sup>[9](https://www.researchgate.net/publication/2206921_Tools_for_event_generator_tuning_and_validation)</sup> |

## Why generators are needed

A high-energy collision such as those at the LHC typically produces hundreds of particles whose momenta range over many orders of magnitude, involving all the particle species of the [Standard Model](https://www.edgepedia.org/standard-model) and possibly some beyond it. The relevant matrix elements are too laborious to compute beyond the first few orders of perturbation theory<sup>[4](https://www.sciencedirect.com/science/article/abs/pii/S0370157311000846)</sup>. At larger distance scales, soft hadronic phenomena such as hadronization and the underlying event cannot be computed from first principles at present, so generators rely on QCD-inspired models<sup>[10](https://pdg.lbl.gov/2022/reviews/rpp2022-rev-mc-event-gen.pdf)</sup>.

Monte Carlo methods make such simulations tractable by drawing random samples. Phase-space integration draws events from a sampling function and estimates the cross section as the average of the event weights; unweighted parton-level events are then generated, and [parton showers](https://www.edgepedia.org/parton-shower), hadronization, and decays are applied on top<sup>[2](https://link.springer.com/article/10.1007/s41781-021-00055-1)</sup>.

## Structure of a simulated event

Generators build up a hadron-hadron collision in several phases<sup>[4](https://www.sciencedirect.com/science/article/abs/pii/S0370157311000846)</sup>:

1. **Hard subprocess.** The primary process of interest, in which heavy objects are created or a large momentum transfer occurs, is generated from matrix elements at leading or next-to-leading QCD perturbative order, sampled over a complicated phase space of kinematic variables<sup>[4](https://www.sciencedirect.com/science/article/abs/pii/S0370157311000846)</sup><sup> • </sup><sup>[1](https://arxiv.org/abs/2509.21611)</sup>.
2. **Parton showers.** Strong and electromagnetic radiation from the incoming and outgoing colored participants is simulated with shower algorithms, which turn inclusive [DGLAP evolution](https://www.edgepedia.org/dglap-evolution) equations into exclusive emission probabilities using Sudakov factors (probability factors suppressing emissions in parton shower algorithms)<sup>[3](https://arxiv.org/abs/2406.08026)</sup>. Historically, parton-shower algorithms were first developed for resummation of collinear singularities<sup>[10](https://pdg.lbl.gov/2022/reviews/rpp2022-rev-mc-event-gen.pdf)</sup>. Parton showers in Herwig 7 come in two complementary varieties: one based on angular ordering, which accounts for QCD coherence and the dead-cone effect affecting heavy quarks, and a dipole-based one designed to work well with NLO matching and multijet merging.<sup>[11](https://arxiv.org/abs/2512.16645)</sup>
3. **Underlying event.** In hadron collisions, additional activity beyond the basic process comes mainly from additional color exchanges between the colliding hadrons, modeled either as multiple perturbative parton-parton interactions (MPI) or nonperturbatively using cut pomerons<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-mc-event-gen.pdf)</sup>.
4. **Hadronization.** The colored partons produced by the shower combine into observable, color-neutral hadrons. Modern generators primarily employ one of two models: the Lund string model, which represents the interaction between color-charged quarks as a color flux tube behaving like a relativistic string, and the cluster model, in which adjacent color lines form primary clusters<sup>[5](https://link.springer.com/article/10.1140/epjc/s10052-025-15182-x)</sup><sup> • </sup><sup>[13](https://www.researchgate.net/publication/268452152_Introduction_to_Parton-Shower_Event_Generators)</sup>.
5. **Decays.** Unstable hadrons and tau leptons decay, in several levels, into the particles measured in detectors, mostly electrons, photons, pions, and kaons<sup>[1](https://arxiv.org/abs/2509.21611)</sup>. Specialized external packages such as EVTGEN for hadron decays and TAUOLA for tau decays provide more sophisticated treatments<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-mc-event-gen.pdf)</sup>.

## Matching and merging with fixed-order calculations

The parton shower approximation alone is accurate only for soft and collinear radiation. Methods developed to combine the shower with fixed-order perturbation theory achieve next-to-leading-order (NLO) accuracy for inclusive observables, making it possible to generate fully simulated hadronic final states with the precision and stability of NLO calculations<sup>[6](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-102711-094928)</sup>. The two principal schemes are MC@NLO and POWHEG, which differ in how they avoid double counting between the matrix element and the shower<sup>[6](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-102711-094928)</sup>. Separately, multijet merging combines tree-level matrix elements for many additional partons with the shower; Sherpa employs the approach of Catani, Krauss, Kuhn, and Webber for this purpose<sup>[14](https://arxiv.org/abs/0811.4622)</sup>.

## The principal generators

Three general-purpose generators dominate current use: Pythia, Herwig, and Sherpa<sup>[3](https://arxiv.org/abs/2406.08026)</sup>.

**Pythia.** The PYTHIA 8.3 manual describes an evolving physics tool most often used to generate high-energy-physics collision events, with a guiding philosophy of reproducing the properties of experimentally obtained collisions as accurately as possible. It covers a wide range of reactions within and beyond the Standard Model, extending to heavy ion physics, with emphasis on phenomena where strong interactions play a major role<sup>[15](https://arxiv.org/abs/2203.11601)</sup>.

**Herwig.** HERWIG 5.1 was already a general-purpose generator covering hard lepton-lepton, lepton-hadron, and hadron-hadron scattering and soft hadron-hadron collisions in one package, using the parton-shower approach for initial-state and final-state QCD radiation<sup>[16](https://www.sciencedirect.com/science/article/pii/0010465592900554)</sup>. Its successor Herwig 7 generates hard processes within the automated Matchbox framework, which supports tree-level, NLO, and loop-induced matrix elements and implements both POWHEG- and MC@NLO-type matching<sup>[11](https://arxiv.org/abs/2512.16645)</sup>.

**Sherpa.** Sherpa is a general-purpose tool for the simulation of particle collisions in lepton-lepton, lepton-photon, photon-photon, lepton-hadron, and hadron-hadron collisions, with a flexible tree-level matrix-element generator for the Standard Model and various new-physics models, a cluster-hadronization model, and QED corrections in the Yennie-Frautschi-Suura approach<sup>[14](https://arxiv.org/abs/0811.4622)</sup><sup> • </sup><sup>[17](https://sherpa-team.gitlab.io/)</sup>.

Experiments combine these tools. The ATLAS collaboration uses Powheg, MadGraph5_aMC@NLO, Sherpa, Pythia, and Herwig for a wide variety of processes; generators implementing higher-order calculations produce parton-level final states that must be interfaced to Pythia or Herwig for the parton shower, hadronization, and particle decays, and afterburners such as Photos++ (QED radiation), Tauola++ (tau decays), and EvtGen (heavy-flavor decays) refine specific aspects<sup>[18](https://doi.org/10.1140/epjc/s10052-024-13701-w)</sup>.

## Interoperability and file formats

The inter-operability of matrix-element providers and event-generation frameworks has been organized through a series of Les Houches accords. The Les Houches Event File (LHEF) format, proposed in 2006 as a minimal XML-style structure, stores process and event information, primarily output from parton-level generators for further use by general-purpose ones<sup>[7](https://arxiv.org/abs/hep-ph/0609017)</sup>. An LHEF file contains an init block with initialization information and one event block per event<sup>[19](https://pythia.org/latest-manual/LHEF.html)</sup>. The Binoth Les Houches Accord provides a runtime interface to one-loop matrix-element providers for matching and merging frameworks including Herwig, Pythia, Sherpa, Powheg Box, and MadGraph5_aMC@NLO<sup>[20](https://cds.cern.ch/record/2900017/files/2131ed685bd7c737a49253b5d96e70f7.pdf)</sup>.

For the output side, HepMC has become the de-facto standard for storing and communicating events at the end of generator simulations<sup>[20](https://cds.cern.ch/record/2900017/files/2131ed685bd7c737a49253b5d96e70f7.pdf)</sup>. HepMC is an object-oriented C++ event record in which particles and vertices are stored in a graph structure reflecting the evolution of the physics event, supporting unlimited entries, color flow, and arbitrary numbers of event weights<sup>[8](https://arxiv.org/abs/1912.08005)</sup><sup> • </sup><sup>[21](https://www.sciencedirect.com/science/article/abs/pii/S0010465500001892)</sup>. The HepMC3 library extends this with simplified event-record structure, arbitrary encoded information, and input-output support for common formats of various generators<sup>[8](https://arxiv.org/abs/1912.08005)</sup>.

## Validation, tuning, and uncertainties

Generators contain phenomenological free parameters that must be fixed against data. The Rivet toolkit validates generator performance against a large library of prominent benchmark analyses, and the Professor system tunes parameters, with its first tunes of Pythia 6 giving substantial improvements on the existing default<sup>[9](https://www.researchgate.net/publication/2206921_Tools_for_event_generator_tuning_and_validation)</sup>. Hadronization parameters may need retuning if significant changes to the decay treatment are made, since the two stages interact through feed-down<sup>[12](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-mc-event-gen.pdf)</sup>.

For a given generator, the prediction can strongly depend on the chosen parameter set, the tune. LHC analyses therefore treat generator choice as a systematic uncertainty: for example, an ATLAS Higgs analysis derived uncertainties by comparing Powheg+Pythia 8 with Sherpa and with Powheg interfaced to Herwig 7<sup>[22](https://doi.org/10.1103%2FPhysRevD.97.072016)</sup>. Comparisons of the three main generators for photoproduction found that all provide a decent description of LEP and HERA jet data within considered uncertainties, while final cross sections show differences related to different non-perturbative inputs<sup>[3](https://arxiv.org/abs/2406.08026)</sup>.

## Computing challenges

Event generation is a major consumer of computing resources, and the High-Luminosity LHC program, expected to be operational from the end of 2027, prompted a review by the HEP Software Foundation Physics Event Generator Working Group of the main software and computing challenges for the generators used by ATLAS and CMS<sup>[2](https://link.springer.com/article/10.1007/s41781-021-00055-1)</sup>. One direction is acceleration of the hard matrix-element calculation itself, as in work on many-gluon tree amplitudes on modern GPUs and a portable parton-level event generator for the high-luminosity LHC<sup>[23](https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.014013)</sup>. Highly parallelized or GPU-accelerated calculations also require more efficient input-output solutions than current event-record formats provide<sup>[20](https://cds.cern.ch/record/2900017/files/2131ed685bd7c737a49253b5d96e70f7.pdf)</sup>.

## References

1. [Jürgen Reuter (2025). Monte Carlo Event Generators. arXiv.](https://arxiv.org/abs/2509.21611)
2. [HSF Physics Event Generator Working Group (2021). Challenges in Monte Carlo Event Generator Software for High-Luminosity LHC. EPJ Research Infrastructures.](https://link.springer.com/article/10.1007/s41781-021-00055-1)
3. [Ilkka Helenius, Peter Meinzinger, Simon Plätzer, Peter Richardson (2024). Photoproduction in general-purpose event generators. arXiv.](https://arxiv.org/abs/2406.08026)
4. [Andy Buckley and colleagues (2011). General-purpose event generators for LHC physics. Physics Reports.](https://www.sciencedirect.com/science/article/abs/pii/S0370157311000846)
5. [Herwig 7 with the Lund string model: tuning and comparative hadronization studies. European Physical Journal C (2025).](https://link.springer.com/article/10.1140/epjc/s10052-025-15182-x)
6. [Paolo Nason, Bryan Webber (2012). Next-to-Leading-Order Event Generators. Annual Review of Nuclear and Particle Science 62:187-213.](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-102711-094928)
7. [J. Alwall and colleagues (2006). A standard format for Les Houches Event Files. arXiv.](https://arxiv.org/abs/hep-ph/0609017)
8. [Andy Buckley and colleagues (2019). The HepMC3 Event Record Library for Monte Carlo Event Generators. arXiv.](https://arxiv.org/abs/1912.08005)
9. [Andy Buckley (2008). Tools for event generator tuning and validation. arXiv.](https://www.researchgate.net/publication/2206921_Tools_for_event_generator_tuning_and_validation)
10. [Particle Data Group (2022). Review 43: Monte Carlo Event Generators.](https://pdg.lbl.gov/2022/reviews/rpp2022-rev-mc-event-gen.pdf)
11. [J. Bellm and colleagues (2025). The Physics of Herwig 7. arXiv.](https://arxiv.org/abs/2512.16645)
12. [Particle Data Group (2024). Review 43: Monte Carlo Event Generators.](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-mc-event-gen.pdf)
13. [Introduction to Parton-Shower Event Generators (2015). ResearchGate.](https://www.researchgate.net/publication/268452152_Introduction_to_Parton-Shower_Event_Generators)
14. [T. Gleisberg and colleagues (2008). Event generation with SHERPA 1.1. arXiv.](https://arxiv.org/abs/0811.4622)
15. [Christian Bierlich and colleagues (2022). A comprehensive guide to the physics and usage of PYTHIA 8.3. arXiv.](https://arxiv.org/abs/2203.11601)
16. [HERWIG 5.1: a Monte Carlo event generator for simulating hadron emission reactions with interfering gluons. Computer Physics Communications 67, 465 (1992).](https://www.sciencedirect.com/science/article/pii/0010465592900554)
17. [Sherpa Homepage, Sherpa documentation.](https://sherpa-team.gitlab.io/)
18. [ATLAS Collaboration (2024). Software and computing for Run 3 of the ATLAS experiment at the LHC. European Physical Journal C.](https://doi.org/10.1140/epjc/s10052-024-13701-w)
19. [Les Houches Event Files, PYTHIA 8.3 manual.](https://pythia.org/latest-manual/LHEF.html)
20. [S. Plätzer (2023). Revisiting event generator, matrix element interfaces and file formats. CERN Document Server.](https://cds.cern.ch/record/2900017/files/2131ed685bd7c737a49253b5d96e70f7.pdf)
21. [The HepMC C++ Monte Carlo event record for High Energy Physics. Computer Physics Communications 134, 41 (2001).](https://www.sciencedirect.com/science/article/abs/pii/S0010465500001892)
22. [ATLAS Collaboration (2018). Search for the standard model Higgs boson produced in association with top quarks. Physical Review D 97, 072016.](https://doi.org/10.1103%2FPhysRevD.97.072016)
23. [Efficient precision simulation of processes with many-jet final states at the LHC. Physical Review D 109, 014013 (2024).](https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.014013)

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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Accelerators and experimental particle physics › Experimental particle physics methods › Monte Carlo simulation in particle physics*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license