Geant4 is a C++ set of libraries designed for Monte-Carlo simultation. This GEometry ANd Tracking toolkit was developped at CERN initially for high energy physics. It contains a large amount of data about particles, interactions and materials inputed according to the standard model. Nowaday, the package is used in a wide range of applications, from DNA modeling to astrophysics.
In order to build a simple simulation, one just has to define a geometry and choose the interactions to consider. For non trivial experiments like \lemu, one may have to personalize some aspects of the original Geant4 code like physics processes or electromagnetic fields. It is now widely used in all kind of fields from dna modelling to astrophysics.
The simulation can be interactive thanks to a user interface already included in Geant4. The possibilities of geometry visualization, histogramming and code personalization are key elements of Geant4.
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