This page contains some documentation to the specific Geant4 implementation of the physics models. Physics processes are collected in physics list. A physics list is a coherent set of processes that covers all physics needs (in terms of primaries, energies and interactions) for a specific user case. A user can create its own physics list, however we recommend to use one of the many physics lists pre-configured and available in Geant4.

In particular LHC experiments converged to use one of the following:

• ATLAS: FTFP_BERT (Fritiof string model at high energy, Bertini cascade at low energy).
• CMS: QGSP_FTFP_BERT_EML (same as QGSP_FTFP_BERT but with a fast electromagnetic physics). Can be used for non-segmented calorimeters.
• LHCb: FTFP_BERT.

Alternatives being investigated include:

• FTFP_BERT_TRV: the transition interval between Fritiof and Bertini is 3-12 GeV (instead of 4-5 GeV of FTFP_BERT).
• QBBC: similar to FTFP_BERT_TRV, but with Binary cascade used for protons and neutrons below 1.5 GeV.

 
Some web-based documentation:

• Reference physics lists
• Detailed description of the physics implementation can be found in Physics Reference Manual

Some notes:

• Parametrized models: it has been shown that the use of parametrized models in the QGSP_BERT physics list creates unphysical discontinuities on the response of calorimeters in the transition region model. Physics lists that contain FTF model relies much less on parametrized models and the issue is strongly suppressed.
• The use of a cascade model is a fundamental ingredient to improve the lateral description of shower shapes.

Disclaimer: This is a simplistic description of Geant4 physics, for a complete description you have to refer to Geant4 website.