Secondary Particles

20.3.1. Secondary Particles#

In the Belle II terminology, primary particles are the ones created by the generators, which are subsequently fed into the Geant4 simulation as the input information.

Secondary particles are the ones created during the Geant4 simulation afterwards. For example, in-flight decays of Ks or Cerenkov lights (i.e., optical photons) created inside the TOP detector are secondary particles.To reduce the size of the output files created by the simulation package, the generator level information on the secondary particles is not stored in the output MCParticle block. This is the default option (note : The life of each secondary particle is still simulated).

If needed, users/software coders are allowed to store the information in the MCParticle block for individual secondary particle by flipping the ignore flag assigned to the particle. Note that it is subdetector group’s responsibility to make sure the MCParticle relation of a hit originated from a secondary particle is assigned in the correct way. Otherwise, the MC track finder routines do not work properly.

The Ignore Flag#

In general, after a new MCParticle (or MCParticleGraph, the internal variable) entry is added, its ignore flag is set to true. The generator level information is blocked from being written in the output file. The exceptions are the following cases. The ignore flag is set to false or should be flipped to false, making their generator level information available in the output file.

  1. Virtual particles. For example, Upsilon(4S).

  2. Primary particles with a simulated Geant4 track.

  3. Decays-in-flight secondary particles with a simulated Geant4 track.

  4. Secondary particles leaving hits in the sensitive detector area.

For Case 4, the flipping operation should be done by each subdetector group. Logically, we can set the flag by directly calling the MCParticleGraph variable such as,

MCParticleGraph.setIgnore(false);

But it may not be easy to find the path to the MCParticleGraph variable when you are located in the sensitive detector area. There is an easier way to set the flag by using the G4Track variable, which is connected to the MCParticleGraph variable via the TrackInfo method internally. For example,

G4Track& track = *step->GetTrack();
Simulation::TrackInfo::getInfo(track).setIgnore(false);

will do the job.

For Cases 1-3, the flag is already set to false in the appropriate places.

MCParticle Relation#

When each subdetector group implements the sensitive detector area, they should decide how to reassign the MCParticle relation of the hits originated from the ignored secondary particles: Either reattribute the element, set the MCParticle Relation weight to zero or a negative value, or drop the relation element completely for the ignored particle.

This is how the MCParticle relation is registered for the hits in the sensitive detector,

registerMCParticleRelation(relation, RelationArray::option);

where the options are:

  • c_doNothing same as before, i.e., reassigns the relation to the parent particle. (This was the default option until 2013 summer).

  • c_zeroWeight set weight to zero and reassigns the relation to the parent particle.

  • c_negativeWeight make weight negative (indicating it is originated from an ignored secondary particle) and reassigns the relation to the parent particle. The default option since 2013 summer.

  • c_deleteElement delete the element

If You Want to Save All (or Most of) the Secondary Particle Information in the MCParticles Block#

From time to time, you may want to save all the secondary particle information in the output MCParticle block for the detector response studies such as shower shape analysis, etc. Here is the way to do as you want. A certain FullSim parameters should be modified to steer the writing process for the secondary particles in the correct way. In the python steering file for your basf2 job script, use the following lines in place of the corresponding add_simulation lines or the equivalent:

g4sim = register_module('FullSim')
g4sim.param('StoreAllSecondaries', True)
g4sim.param('SecondariesEnergyCut', 1.0)
path.add_module(g4sim)

Since there could be too many very low energy secondaries which may blow up the size of the MCParticles block into an uncontrollable number, we have an additional parameter called SecondaryEnergyCut. The default value is 1.0 MeV. If the energy of a secondary particle is below this threshold, the particle information will not be saved in the MCParticles block, even though the StoreAllSecondaries parameter is set to True. Otherwise, if you do not mind the size of the MCParticles block being too large, you can set this threshold parameter as 0.

Physics Process#

The Geant4 provides the information on via which physics process (and its subtype) a secondary particle is created. This is stored in the MCParticles and can be retrieved as

MCParticle.getSecondaryPhysicsProcess();

Between the physics process type and subtype, the subtype parameter gives more detailed information. What is implemented in basf2 is the subtype information. Unfortunately, the Geant4 people did not store the definition of subtypes in one file, so one should check several Geant4 files for these numbers. The following is the list of constants implemented in Geant4 v9.6. They used the same list for v10, too.

Table 20.1 Physics Process and its subtype#

enum G4EmProcessSubType

enum G4HadronicProcessType

enum G4DecayProcessType

fCoulombScattering = 1

fHadronElastic = 111

DECAY = 201

fIonisation = 2

fHadronInelastic = 121

DECAY_WithSpin,

fBremsstrahlung = 3

fCapture = 131

DECAY_PionMakeSpin,

fPairProdByCharged = 4

fMuAtomicCapture = 132

DECAY_Unknown = 211

fAnnihilation = 5

fFission = 141

DECAY_MuAtom = 221

fAnnihilationToMuMu = 6

fHadronAtRest = 151

DECAY_External = 231

fAnnihilationToHadrons = 7

fLeptonAtRest = 152

fNuclearStopping = 8

fChargeExchange = 161

fElectronGeneralProcess = 9

fRadioactiveDecay = 210

fMultipleScattering = 10

fRayleigh = 11

fPhotoElectricEffect = 12

fComptonScattering = 13

fGammaConversion = 14

fGammaConversionToMuMu = 15

fGammaGeneralProcess = 16

fCerenkov = 21

fScintillation = 22

fSynchrotronRadiation = 23

fTransitionRadiation = 24

Release note (newly included parameters since) :

  • G4DecayProcessType : fElectronGeneralProcess and fGammaGeneralProcess since v10.5

  • G4HadronicProcessType : fMuAtomicCapture and fLeptonAtRest since v10.4

  • G4DecayProcessType : DECAY_MuAtom since v10.4

Note

  • If the MCParticles entry is a primary particle, “0” is assigned.

  • When the Geant4 does not give the physics process subtype information, “-1” is assigned. This happens rarely.

Warning

fElectronGeneralProcess and fGammaGeneralprocess are internal Geant4 parameters, which general users are not supposed to see. If you see any of these, please contact the simulation convener or Geant4 team.

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