Les Houches Accord

Les Houches Accord
Name	Les Houches Accord
Established	1990s
Location	Les Houches

Les Houches Accord.

The Les Houches Accord is a set of community-driven agreements originating from the Les Houches Summer School meetings that established conventions for data interchange between high energy physics software tools and collaborations such as CERN, SLAC National Accelerator Laboratory, Fermilab, DESY, and INFN. The Accord formalized interfaces used by projects like PYTHIA, HERWIG, MadGraph, SOFTSUSY, and GEANT4 to enable interoperability among researchers from institutions including University of Oxford, Massachusetts Institute of Technology, École Normale Supérieure, Harvard University, and University of California, Berkeley.

Background and History

The origins trace to workshops at Les Houches that brought together developers from LHC experiments, theorists from SLAC, IHEP, and tool authors associated with IPPP and CEA Saclay to address incompatibilities between event generators such as ISAJET, HERWIG, and PYTHIA. Early milestones included the first accord proposals discussed alongside working groups from LEP, Tevatron, and HERA communities, influenced by standards used at CERN computing centers and practices from Monte Carlo methods groups collaborating with Institut des Hautes Études Scientifiques and Max Planck Institute for Physics.

Purpose and Scope

The Accord aimed to define file formats and metadata descriptors to exchange parton-level events, particle spectra, and decay information among tools like MadGraph, CalcHEP, CompHEP, and analysis frameworks such as ROOT. Its scope encompassed interfaces for spectrum calculators used in supersymmetry phenomenology from groups including SPheno, ISAJET, and SOFTSUSY, while facilitating detector simulation inputs for GEANT4, Delphes, and experiment-specific frameworks used by ATLAS and CMS. The conventions sought to align practices between collaborations at CERN, FNAL, DESY, KEK, and theoretical consortia at IPPP and KITP.

Technical Specifications and Standards

Technical work specified card formats, block conventions, and parameter naming inspired by existing schemes in HEPData and proposals from Les Houches workshops; these include standardized blocks for model parameters, particle PDG codes, and decay tables compatible with Particle Data Group conventions. The specifications addressed translation between ASCII-based spectra, XML-derivative structures, and binary payloads usable by ROOT I/O and serialization libraries from Boost employed by C++-based generators. The standards also recommended validation suites leveraging test cases from MadGraph5_aMC@NLO, cross checks with MCFM, and comparisons to prediction sets used by ATLAS and CMS analysis groups.

Implementations and Software Support

The Accord was implemented in major codebases: parsers and writers were added to PYTHIA, HERWIG, MadGraph, CalcHEP, CompHEP, SPheno, SOFTSUSY, and analysis tools such as ROOT macros maintained by teams at CERN and LAL. Library support appeared in community projects hosted by GitHub and mirrored by institutional repositories at CERN GitLab, with continuous integration testing using services similar to those adopted by Travis CI and Jenkins setups at research centers including Brookhaven National Laboratory and Argonne National Laboratory.

Impact and Adoption in Particle Physics

Widespread adoption of the Accord accelerated interoperability across collaborations like ATLAS, CMS, LHCb, and ALICE, enabling consistent phenomenological studies from groups at Princeton University, University of Cambridge, University of Tokyo, and Imperial College London. It facilitated global analyses combining results from LEP legacy datasets, Tevatron measurements, and LHC searches, informing theoretical efforts at institutes such as Perimeter Institute and KIT. The Accord also influenced publication pipelines at journals like Physical Review Letters and Journal of High Energy Physics by streamlining the reproducibility of generator-level predictions submitted by authors from collaborations including ATLAS and CMS.

Related Agreements and Successors

Following the original Accord, related initiatives and successors emerged such as the HepMC format, the SLHA family for supersymmetric models, and proposals developed during subsequent Les Houches workshops that interfaced with standards like HEPMC3 and metadata schemas adopted by HEPData. Later community efforts coordinated by CERN working groups, IPPP, and collaborations among FNAL and DESY produced updates and complementary accords influencing software stacks including MadGraph5_aMC@NLO, Sherpa, and Whizard.

Category:High energy physics software