KAHIBARO
12.1. What Is a Physics List?
Table of Contents
Particle definitions
A Geant4 physics list is the part of your application that tells Geant4 which particles exist in your simulation and how they behave. At the most basic level, it is a collection of C++ classes that register particle types, attach physical processes to those particles, and control how interactions are simulated during tracking.
The first role of a physics list is to define the set of particles that can appear. Each particle in Geant4 is represented by a class derived from G4ParticleDefinition. For a beginner application you almost never create these definitions yourself. Instead you request standard particles from Geant4 and the reference physics lists do this automatically.
A typical physics list will include at least the following categories of particles: gamma, electrons, positrons, and often protons and neutrons. More complete lists also include pions, kaons, ions, and short lived particles. When you choose a reference physics list such as FTFP_BERT, it internally instantiates the necessary particle classes so that they can be used as primaries or created as secondaries during interactions.
The choice of particle content has a direct effect on what your simulation can produce. If a particle type is not defined in the physics list, Geant4 will not track it, even if a process would in reality create it. For example, if you only use an electromagnetic physics list that defines electrons, positrons, and gammas, then hadronic particles such as pions will not appear, because they are neither defined nor associated with any process.
You can extend the particle content by adding Geant4 modular physics constructors in your own physics list. For instance, you might add a class that defines ions if you plan to simulate heavy ion beams. The core idea remains that the physics list is the single place where Geant4 learns which particle species are allowed to exist and participate in the simulation.
A particle that is not defined in the physics list cannot be created, tracked, or interact in the simulation.
Physical processes
Once the particles exist, the physics list must specify which physical processes act on each particle. A physical process in Geant4 represents a specific type of interaction, such as ionization of matter by charged particles, Compton scattering of photons, or elastic scattering of neutrons.
Each process is implemented as a C++ class derived from G4VProcess. The physics list attaches selected process objects to each particle type. For example, electrons are usually given ionization, bremsstrahlung, and multiple scattering processes. Photons are given photoelectric effect, Compton scattering, and pair production, among others. Without this process registration, particles would simply fly through materials without losing energy or changing direction.
The physics list also controls the numerical models that are used inside each process. Many processes have several possible models that cover different energy ranges or provide different accuracy and performance. A reference physics list chooses reasonable combinations of models and energy ranges so you do not need to assemble them by hand for common applications.
In practice, this means that by selecting a physics list you are selecting both the set of processes and the level of detail in the underlying physics. A low energy electromagnetic list, for example, uses models that describe atomic shell structure and detailed cross sections at low energies. A hadronic list determines how protons and neutrons interact with nuclei, including nuclear reactions and secondary particle production.
The physics list also determines when each process can act during tracking. Processes are classified as continuous, discrete, or at-rest, and the list registers them accordingly. Continuous processes, such as ionization energy loss, act along the step. Discrete processes, such as a photon interaction, occur at a specific point. At-rest processes act when a particle has come to rest, such as radioactive decay for a stopped nucleus.
Only processes that are explicitly registered for a particle in the physics list can act on that particle. Missing processes lead to unphysical behavior, such as no energy loss, no scattering, or no secondary production.
Particle transport
The third key role of a physics list is to control how particles are transported through materials by combining particle definitions and processes into a consistent tracking scheme. Transport here means how Geant4 advances a particle step by step, applies energy loss and scattering, and decides when an interaction happens or when a track is terminated.
During transport, Geant4 evaluates all processes attached to a particle to determine the possible interactions along its path. Each discrete process provides an interaction length, which is related to the probability of an interaction in the next segment of the trajectory. Continuous processes provide rates of energy loss or angular deflection per unit length. The transport engine then chooses a step length that respects all these constraints, including geometry boundaries, and updates the particle state accordingly.
The physics list affects transport through several mechanisms. It sets the processes themselves, which define cross sections and stopping powers. It also sets production cuts, which are thresholds that control when secondary particles are actually created and tracked instead of their effect being approximated. For example, a higher production cut for gammas will result in fewer low energy secondary photons, making transport faster but less detailed.
Transport accuracy and performance are closely linked to physics list choices. A more detailed physics list usually includes more processes and more complex models, with finer control of secondary production. This yields more realistic transport, particularly for low energies or in complicated materials, at the cost of longer computation times. A simpler list reduces the number of interactions and the detail of transport, which can be acceptable for some applications where such details are not critical.
From the user perspective, the essential point is that choosing a physics list is equivalent to choosing how Geant4 will move particles, how often they interact, what secondaries are generated, and how finely the simulation follows their path. When you select a reference physics list in your application, you are selecting a complete transport scheme that has been tuned and validated for typical use cases.
The physics list directly controls transport accuracy and performance. Inappropriate physics list choices can lead to biased step lengths, missing secondaries, or unphysical ranges and energy deposition patterns.
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