HepSim  is a public repository for Monte Carlo event generators for particle physics experiments. It includes data samples with outputs of Monte Carlo event generators, typically generated at leading-order matrix elements of QCD, as well as at fixed orders. The HepSim repository is created at the Argonne National Laboratory, leveraging its computational resources, as well as the resources from the grid computing managed by the Open Science Grid Consortium. The project was designed to publish large Monte Carlo event samples for physics and detector performance studies for physics community without authentication, which is usually required for the grid computing resources. The web interface of this repository is mirrored at NERSC, CERN and JLab.
The repository allows a quick reconstruction of kinematic distribution using experiment-specific selections and kinematic cuts. The usefulness of the repository for storing files with Monte Carlo simulations is justified by the fact that the processing time for analysis of such files is relatively small while, in many cases, the CPU time to generate event samples can be substantial. Another goal of HepSim is a long-term preservation of theoretical predictions and a possibility to have reference event-based predictions for experimentalists and theorists. For example, common samples with event generator outputs can be useful for comparison between different collider experiments that often use different selection cuts.
The repository was used to create one of the largest Monte Carlo event sample with open access  . It was also used as a source for complex detector simulations  using Geant4 program. A full list of publications that used the repository is available on the HepSim documentation web site.
The main approach of the HepSim repository is to store events in the form of highly-compressed ProMC data format  using the http access. This data format uses varints as implemented in the Google's protocol buffer library. The HepSim data can be read and analyzed using several programming languages and libraries, such as C++ / ROOT program and Java/DataMelt. In addition, a number of light-weight software tools are provided by the HepSim repository itself .
History and usage
The HepSim repository was originally started as a project to deliver Monte Carlo generator samples for the particle-physics community during the US long-term planning study at the Snowmass DPF meetings of the American Physical Society’s Division of Particles and Fields in 2013. The original HepSim paper  was published in 2014. After the Snowmass, the main focus has been shifted from fast-detector simulations to truth-level events for future proton-proton colliders, as well as to the full detector simulations. In particular, HepSim had an emphasis  on a 100 TeV proton-proton collider . The repository is also discussed in the context of the electron-ion detector concepts.