5.3.  Particles

5.3.1.  Basic concepts

There are three levels of classes to describe particles in Geant4.

defines a particle
describes a particle interacting with materials
describes a particle traveling in space and time

G4ParticleDefinition aggregates information to characterize a particle's properties, such as name, mass, spin, life time, and decay modes. G4DynamicParticle aggregates information to describe the dynamics of particles, such as energy, momentum, polarization, and proper time, as well as ``particle definition'' information. G4Track (see Section 5.1) includes all information necessary for tracking in a detector simulation, such as time, position, and step, as well as ``dynamic particle'' information.

5.3.2.  Definition of a particle

There are a large number of elementary particles and nuclei. Geant4 provides the G4ParticleDefinition class to represent particles, and various particles, such as the electron, proton, and gamma have their own classes derived from G4ParticleDefinition.

We do not need to make a class in Geant4 for every kind of particle in the world. There are more than 100 types of particles defined in Geant4 by default. Which particles should be included, and how to implement them, is determined according to the following criteria. (Of course, the user can define any particles he wants. Please see the User's Guide: For ToolKit Developers).  Particle List in Geant4

This list includes all particles in Geant4 and you can see properties of particles such as

  • PDG encoding
  • mass and width
  • electric charge
  • spin, isospin and parity
  • magnetic moment
  • quark contents
  • life time and decay modes

Here is a list of particles in Geant4. This list is generated automatically by using Geant4 functionality, so listed values are same as those in your Geant4 application (as far as you do not change source codes).

Categories  Classification of particles

  1. elementary particles which should be tracked in Geant4 volumes

    All particles that can fly a finite length and interact with materials in detectors are included in this category. In addition, some particles with a very short lifetime are included for user's convenience.

    1. stable particles

      Stable means that the particle can not decay, or has a very small possibility to decay in detectors, e.g., gamma, electron, proton, and neutron.

    2. long life (>10-14sec) particles

      Particles which may travel a finite length, e.g., muon, charged pions.

    3. short life particles that decay immediately in Geant4

      For example, pi0, eta

    4. K0 system

      K0 "decays" immediately into K0S or K0L, and then K0S/ K0L decays according to its life time and decay modes.

    5. optical photon

      Gamma and optical photon are distinguished in the simulation view, though both are the same particle (photons with different energies). For example, optical photon is used for Cerenkov light and scintillation light.

    6. geantino/charged geantino

      Geantino and charged geantino are virtual particles for simulation which do not interact with materials and undertake transportation processes only.

  2. nuclei

    Any kinds of nucleus can be used in Geant4, such as alpha(He-4), uranium-238 and excited states of carbon-14. In addition, Geant4 provides hyper-nuclei. Nuclei in Geant4 are divided into two groups from the viewpoint of implementation.

    1. light nuclei

      Light nuclei frequently used in simulation, e.g., alpha, deuteron, He3, triton.

    2. heavy nuclei (including hyper-nuclei)

      Nuclei other than those defined in the previous category.

    3. light anti-nuclei

      Light anti-nuclei for example anti-alpha.

    Note that G4ParticleDefinition represents nucleus state and G4DynamicParticle represents atomic state with some nucleus. Both alpha particle with charge of +2e and helium atom with no charge aggregates the same "particle definition" of G4Alpha, but different G4DynamicParticle objects should be assigned to them. (Details can be found below)

  3. short-lived particles

    Particles with very short life time decay immediately and are never tracked in the detector geometry. These particles are usually used only inside physics processes to implement some models of interactions. G4VShortLivedParticle is provided as the base class for these particles. All classes related to particles in this category can be found in shortlived sub-directory under the particles directory.

    1. quarks/di-quarks: For example, all 6 quarks.
    2. gluon
    3. baryon excited states with very short life: For example, spin 3/2 baryons and anti-baryons
    4. meson excited states with very short life: For example, spin 1 vector bosons  Implementation of particles

Single object created in the initialization : Categories a, b-1

These particles are frequently used for tracking in Geant4. An individual class is defined for each particle in these categories. The object in each class is unique. The user can get pointers to these objects by using static methods in their own classes. The unique object for each class is created when its static method is called in the ``initialization phase''.

On-the-fly creation: Category b-2

Ions will travel in a detector geometry and should be tracked, however, the number of ions which may be used for hadronic processes is so huge that ions are dynamically created by requests from processes (and users). Each ion corresponds to one object of the G4Ions class. G4IonTable class is a dictionary for ions. G4IonTable::GetIon() method to create ions on the fly. (G4IonTable::FIndIon() method returns pointer to the specified ion. If the ion does not exists, it returns zero without creating any ion.

G4NucleiPropertiesTableAME03 contains a table of mesaured mass values of about 3100 stable nuclei (ground states). G4NucleiPropertiesTheoreticalTable theoretical mass values of about 8000 nuclei (ground states). G4IsotopeTable describes properties of ions (exited energy, decay modes, life time and magnetic moments), which are used to create ions. G4NuclideTable is provided as a list of nuclei in Geant4. It contains about 2900 ground states and 4000 excited states. Users can register his/her G4IsotopeTable to the G4IonTable.

Processes attached to heavy ions are same as those for G4GenericIon class. In other words, you need to create G4GenericIon and attach processes to it if you want to use heavy ions.

G4ParticleGun can shoot any heavy ions with /gun/ions command after ``ion'' is selected by /gun/particle command.

Dynamic creation by processes: Category c

Particle types in this category are are not created by default, but will only be created by request from processes or directly by users. Each shortlived particle corresponds to one object of a class derived from G4VshortLivedParticle, and it will be created dynamically during the ``initialization phase''.  G4ParticleDefinition

The G4ParticleDefinition class has ``read-only'' properties to characterize individual particles, such as name, mass, charge, spin, and so on. These properties are set during initialization of each particle. Methods to get these properties are listed in Table 5.2.

G4String GetParticleName() particle name
G4double GetPDGMass() mass
G4double GetPDGWidth() decay width
G4double GetPDGCharge() electric charge
G4double GetPDGSpin() spin
G4double GetPDGMagneticMoment() magnetic moment (0: not defined or no magnetic moment)
G4int GetPDGiParity() parity (0:not defined)
G4int GetPDGiConjugation() charge conjugation (0:not defined)
G4double GetPDGIsospin() iso-spin
G4double GetPDGIsospin3() 3rd-component of iso-spin
G4int GetPDGiGParity() G-parity (0:not defined)
G4String GetParticleType() particle type
G4String GetParticleSubType() particle sub-type
G4int GetLeptonNumber() lepton number
G4int GetBaryonNumber() baryon number
G4int GetPDGEncoding() particle encoding number by PDG
G4int GetAntiPDGEncoding() encoding for anti-particle of this particle

Table 5.2.  Methods to get particle properties.

Table 5.3 shows the methods of G4ParticleDefinition for getting information about decay modes and the life time of the particle.

G4bool GetPDGStable() stable flag
G4double GetPDGLifeTime() life time
G4DecayTable* GetDecayTable() decay table

Table 5.3.  Methods to get particle decay modes and life time.

Users can modify these properties, though the other properties listed above can not be change without rebuilding the libraries.

Each particle has its own G4ProcessManger object that manages a list of processes applicable to the particle.(see Section 2.5.2 )

5.3.3.  Dynamic particle

The G4DynamicParticle class has kinematics information for the particle and is used for describing the dynamics of physics processes. The properties in G4DynamicParticle are listed in Table 5.4.

G4double theDynamicalMass dynamical mass
G4ThreeVector theMomentumDirection normalized momentum vector
G4ParticleDefinition* theParticleDefinition definition of particle
G4double theDynamicalSpin dynamical spin (i.e. total angular momentum as a ion/atom )
G4ThreeVector thePolarization polarization vector
G4double theMagneticMoment dynamical magnetic moment (i.e. total magnetic moment as a ion/atom )
G4double theKineticEnergy kinetic energy
G4double theProperTime proper time
G4double theDynamicalCharge dynamical electric charge (i.e. total electric charge as a ion/atom )
G4ElectronOccupancy* theElectronOccupancy electron orbits for ions

Table 5.4.  Methods to set/get values.

Here, the dynamical mass is defined as the mass for the dynamic particle. For most cases, it is same as the mass defined in G4ParticleDefinition class ( i.e. mass value given by GetPDGMass() method). However, there are two exceptions.

  • resonance particle
  • ions

Resonance particles have large mass width and the total energy of decay products at the center of mass system can be different event by event.

As for ions, G4ParticleDefintion defines a nucleus and G4DynamicParticle defines an atom. G4ElectronOccupancy describes state of orbital electrons. So, the dynamic mass can be different from the PDG mass by the mass of electrons (and their binding energy). In addition, the dynamical charge, spin and magnetic moment are those of the atom/ion (i.e. including nucleus and orbit electrons).

Decay products of heavy flavor particles are given in many event generators. In such cases, G4VPrimaryGenerator sets this information in *thePreAssignedDecayProducts. In addition, decay time of the particle can be set arbitrarily time by using PreAssignedDecayProperTime.