Geant4 User's Guide for Application Developers - Geant4 - CERN

Tracking and PhysicsIdle> /particle/select pi0Idle> /particle/property/decay/select 0Idle> /particle/property/decay/br 0Idle> /particle/property/decay/select 1Idle> /particle/property/decay/br 1Idle> /particle/property/decay/dumpG4DecayTable: pi00: BR: 0 [Phase Space] : gamma gamma1: BR: 1 [Dalitz Decay] : gamma e- e+5.2.3.3. Pre-assigned Decay Modes by Event GeneratorsDecays of heavy flavor particles such as B mesons are very complex, with many varieties of decay modes anddecay mechanisms. There are many models for heavy particle decay provided by various event generators and itis impossible to define all the decay modes of heavy particles by using G4VDecayChannel. In other words, decaysof heavy particles cannot be defined by the Geant4 decay process, but should be defined by event generators orother external packages. Geant4 provides two ways to do this: pre-assigned decay mode and externaldecayer.In the latter approach, the class G4VExtDecayer is used for the interface to an external package which definesdecay modes for a particle. If an instance of G4VExtDecayer is attached to G4Decay, daughter particles will begenerated by the external decay handler.In the former case, decays of heavy particles are simulated by an event generator and the primary event containsthe decay information. G4VPrimaryGenerator automatically attaches any daughter particles to the parent particleas the PreAssignedDecayProducts member of G4DynamicParticle. G4Decay adopts these pre-assigned daughterparticles instead of asking G4VDecayChannel to generate decay products.In addition, the user may assign a pre-assigned decay time for a specific track in its rest frame (i.e. decay timeis defined in the proper time) by using the G4PrimaryParticle::SetProperTime() method. G4VPrimaryGeneratorsets the PreAssignedDecayProperTime member of G4DynamicParticle. G4Decay uses this decay time instead ofthe life time of the particle type.5.2.4. Photolepton-hadron ProcessesTo be delivered.5.2.5. Optical Photon ProcessesA photon is considered to be optical when its wavelength is much greater than the typical atomic spacing. InGEANT4 optical photons are treated as a class of particle distinct from their higher energy gamma cousins. Thisimplementation allows the wave-like properties of electromagnetic radiation to be incorporated into the opticalphoton process. Because this theoretical description breaks down at higher energies, there is no smooth transitionas a function of energy between the optical photon and gamma particle classes.For the simulation of optical photons to work correctly in GEANT4, they must be imputed a linear polarization.This is unlike most other particles in GEANT4 but is automatically and correctly done for optical photons that aregenerated as secondaries by existing processes in GEANT4. Not so, if the user wishes to start optical photons asprimary particles. In this case, the user must set the linear polarization using particle gun methods, the GeneralParticle Source, or his/her PrimaryGeneratorAction. For an unpolarized source, the linear polarization should besampled randomly for each new primary photon.The GEANT4 catalogue of processes at optical wavelengths includes refraction and reflection at medium boundaries,bulk absorption and Rayleigh scattering. Processes which produce optical photons include the Cerenkov effect,transition radiation and scintillation. Optical photons are generated in GEANT4 without energy conservationand their energy must therefore not be tallied as part of the energy balance of an event.The optical properties of the medium which are key to the implementation of these types of processes are storedas entries in a G4MaterialPropertiesTable which is linked to the G4Material in question. Theseproperties may be constants or they may be expressed as a function of the photon's wavelength. This table is165