Brief Overview of the Geant4 Physics Models

Electromagnetic interactions of photons and charged particles with matter are implemented in two electromagnetic packages. The Standard EM package is tailored for applications where secondaries only above 1KeV are important. For applications where lower energy secondaries, below 1KeV, and additional effects, such as atomic relaxation and shell effects, are important, the Low Energy EM package is provided.

For hadronic interactions, Geant4 includes scross sections and physics models from thermal energies (for neutrons). For many regimes, a choice of physics models is available, enabling users to choose between more precision and better CPU performance.
At high energies (E>20GeV) the Quark-Gluon String (QGS) model and a Fritiof-like String (FTF) model provide theory-driven interaction models of the initial projectile-nucleon collision.
At energies below ~10GeV, two cascade models are provided, one following the Bertini approach, and one, called Binary cascade, which is more theory-based.
A number of models is available to perform the de-excitaion of the nucleus that remains after a high energy collision.
One of them is the precompound model which is currently used as a "back-end" for the QGS and Binary cascade models (the Bertini-like cascade has its own evaporation model).
The slowest remaining particles may also be re-absorbed by the nucleus. They may be treated by the Chiral Invariant Phase space (CHIPS) model, which may also be used as a back-end for the QGS model.
For low energy projectiles (<20MeV), high-precision neutron processes and photo-evaporation codes are provided; each relies on the data library distributed with Geant4.
For photo-nuclear and electro-nuclear interactions processes are provided that use the CHIPS model. Stopping particles are treated with the use of CHIPS-based capture process.
For all long-lived particles at all energies there are Low Energy Parametrized (LEP) and High Energy Parametrized (HEP) models which originate from the old GHEISHA hadronic package of Geant3. The LEP models covers the transition region between the cascade models and the string models (from 10 to 20GeV). The LEP and HEP model provides a faster alternative to the theory-driven models. Both LEP and HEP models are also used for some long-lived baryons, due to the lack of alternatives.