GenEvent

GenEvent is a data structure used in high-energy physics to represent a single generated collision event in Monte Carlo simulations. It is a central component of the HepMC family of event record libraries and is designed to enable seamless exchange of event information between event generators, detector simulations, and analysis software.

A GenEvent contains the complete final-state and intermediate-state information for one simulated collision. Its core elements are GenParticle objects, which hold properties such as four-momenta, mass, PDG particle codes, and status codes, and GenVertex objects that encode production and decay vertices. The particles are linked by mother-daughter relationships to form the event graph, allowing traversal of production and decay chains.

In addition to particle data, a GenEvent stores event-level metadata. This can include the collision energy, beam particles, event number, and optional weights or cross-section information. The metadata provides essential context for analysis and reproducibility.

GenEvent is designed for interoperability with multiple formats and workflows. It can be produced by various external generators (e.g., Pythia, Herwig, Sherpa) and exchanged through formats such as HEPEVT-era blocks, Les Houches Event File (LHEF), or HepMC’s own formats (ASCII, binary, or ROOT-based). This flexibility supports integration with detector simulations (such as Geant4) and with analysis pipelines that require consistent event representations.

Overall, GenEvent serves as the standard in its ecosystem for storing and passing generated events, enabling researchers to analyze particle interactions, validate models, and study detector responses in a coherent, traceable workflow. Variants exist across HepMC versions (notably HepMC2 and HepMC3), reflecting different design choices in data organization and interfaces.