GEANT4

Geant4 is a software toolkit for the simulation of the passage of particles through matter. It is open-source, cross-platform, and written in C++. Developed by an international collaboration coordinated by CERN and INFN, Geant4 provides a general framework for describing geometries, materials, particle interactions, and the transport of particles through complex detector setups and environments.

It is used in high-energy physics, medical physics, radiation protection, and space science to model how particles interact with matter and how detectors respond. The toolkit offers a modular set of physics models covering electromagnetic, hadronic, and optical processes, along with configurable geometry, materials, and magnetic fields. It supports detailed simulation of tracking, energy deposition, scattering, and secondary particle production, with capabilities for both high-precision and faster approximate simulations, as well as sensitivity studies.

Geant4's architecture is modular. Core components include geometry construction and navigation, including volumes and materials; particle definitions and physics processes; a physics list that selects models for different particle types and energy ranges; the tracking and stepping algorithms; event and run management; primary particle generation; and user actions. It also provides visualization and graphical user interfaces, and supports parallel processing and multi-threading in recent releases.

Geant4 evolved from the Geant3 program in the 1990s as a re-engineering into an object-oriented, highly extensible toolkit. It has become a standard tool for detector simulation in many research areas, with widespread adoption by major experiments and institutions worldwide, and ongoing development to incorporate new physics models, geometry features, and performance improvements.