HEPMC

HEPMC
Name	HEPMC
Developer	CERN; contributions from ATLAS (experiment), CMS (experiment), LHCb, ALICE (A Large Ion Collider Experiment)
Released	1997
Latest release	2.x series
Programming language	C++
Operating system	Linux, macOS, Windows
License	GNU General Public License

HEPMC

HEPMC is an open-source C++ library and file format used for representing simulated high-energy particle collision events. It provides a standardized in-memory and on-disk representation that interoperates with event generators, detector simulation frameworks, and analysis tools developed at facilities such as CERN and collaborations like ATLAS (experiment), CMS (experiment), LHCb, and ALICE (A Large Ion Collider Experiment). HEPMC bridges event generators such as Pythia, Herwig, and Sherpa with analysis environments used in experiments including ATLAS (experiment), CMS (experiment), and theory tools at institutions like DESY and Fermilab.

Introduction

HEPMC was created to standardize the exchange of Monte Carlo event records between generator programs and downstream consumers. It encapsulates hard-process information, parton shower history, hadronization results, and event weights in a graph-like structure compatible with ROOT, Geant4, and generator outputs from Pythia, Herwig, Sherpa, MadGraph5_aMC@NLO, and Powheg. HEPMC supports provenance metadata referencing experiments and computing projects such as WLCG and data-management systems at CERN and national laboratories including SLAC National Accelerator Laboratory and Brookhaven National Laboratory.

History and Development

HEPMC originated in the late 1990s as the community sought a flexible event record beyond legacy formats used by experiments like ALEPH, DELPHI, and OPAL at the Large Electron–Positron Collider. Early development involved contributors from CERN, DESY, and the LHC collaborations to accommodate generators including Pythia 6 and Herwig 6. The format evolved through major updates to support next-to-leading-order generators such as MC@NLO and Powheg Box, and to integrate with modern analysis stacks exemplified by ROOT and Rivet. Maintenance and releases have been coordinated with computing initiatives at CERN and community projects like HEPData and the HepForge hosting service.

Architecture and Data Model

HEPMC models events as a directed acyclic graph of vertices and particles. Each vertex connects incoming and outgoing particles, mirroring interaction points used in theoretical descriptions found in works by Feynman and implemented in generator frameworks like MadGraph5_aMC@NLO and CompHEP. Particle objects carry PDG identifiers standardized by the Particle Data Group, four-momenta consistent with conventions used in PDG (Particle Data Group) listings, status codes aligned with generators such as Pythia and Herwig, and optional attributes for polarization, color flow, and event weights. The event container stores run-level metadata including generator settings, cross-sections, and random-seed provenance used by services like GRIDPP and middleware developed in Worldwide LHC Computing Grid collaborations. The design allows serialization to multiple file formats and conversion to analysis-ready structures for frameworks like ROOT and validation tools such as Rivet.

File Formats and Interfaces

HEPMC historically provided ASCII and binary persistent representations; modern iterations emphasize compactness and interoperability with formats like ROOT trees and HDF5-backed pipelines used in projects at CERN and large experiments. Interfaces exist for direct integration with generators including Pythia, Herwig, Sherpa, MadGraph5_aMC@NLO, and Powheg Box via adapter layers. Bindings and converters facilitate transformation between HEPMC and older standards used by experiments such as CDF and D0, as well as interoperability with detector simulation toolkits like Geant4 and analysis environments at ATLAS (experiment) and CMS (experiment). Event weight handling supports schemes used in precision computations from collaborations like NNPDF and tools such as MCFM.

Usage and Applications

HEPMC underpins workflows across phenomenology, detector simulation, and experimental analysis. The format is used to pass events from generator suites like Pythia, Herwig, and Sherpa into detector simulators such as Geant4 and full reconstruction chains in ATLAS (experiment) and CMS (experiment). It is central to validation work with tools like Rivet and comparisons against results archived at HEPData. Phenomenologists employ HEPMC for studies with matrix-element generators including MadGraph5_aMC@NLO, CalcHEP, and CompHEP, and to feed parton distribution function fits referencing datasets from NNPDF and CTEQ. HEPMC records are also used in outreach and education via event displays and visualization tools developed in communities around CERN and national laboratories like Fermilab.

Tools and Ecosystem

A broad ecosystem surrounds HEPMC: converters and utilities hosted on platforms such as HepForge and integrated into continuous-integration pipelines at CERN; analysis adapters with ROOT and plotting tools used by ATLAS (experiment) and CMS (experiment); validation suites like Rivet and generator interface projects maintained by collaborations including the LHCb Collaboration and ALICE (A Large Ion Collider Experiment). Bindings and wrappers exist for scripting environments popular in the community, and packaging is provided by distributions and package managers used at CERN and national computing centers. Community governance and development follow collaborative models similar to other HEP infrastructure projects like Geant4 and ROOT.

Category:High energy physics software