-- IvantchenkoV - 11 Aug 2014

CMSSW MC Truth Guide

Introduction

During Geant4 simulation huge number of tracks are transported via CMS. Only small fraction of these tracks are kept as transient G4SimTrack objects, also small number of transient G4SimVertex objects are kept. For persistency SimVertex and SimTrack objects are filled (there one to one correspondence, no filter applied). User manual is provided in the following twiki. Another MC truth information is inside PSimHits objects. In this page a description of MC truth objects is provided.

G4SimTrack and G4SimVertex

G4SimVertex includes:

  • global position
  • global time
  • index of parent track
  • Geant4 process type responsible for the vertex

G4SimTrack includes:

  • momentum
  • energy
  • track ID
  • index of generator particle
  • index of G4SimVertex
  • index of parent particle
  • parent momentum
  • position at tracker surface
  • momentum at tracker surface

SimTrack and SimVertex

SimVertex includes:

  • event ID
  • 4-vector position + time
  • parent track index
  • vertex index
  • process type

SimTrack includes:

  • event ID
  • track ID
  • PDG code
  • 4-momentum
  • vertex index
  • generator particle index
  • track position at tracker surface
  • 4-momentum at tracker surface

Geant4 process numbering scheme

Geant4 process Type 5_3_X Type 7_X_Y
Primary generator 100 0
Transportation 0 91
Coulpled transportation 0 92
Coulomb scattering 0 1
Ionisation 51-54 2
Bremsstrahlung 56, 61, 66 3
e+e- pair production 62, 67 4
Annihilation in 2 gamma 0 5
Annihilation in mu+mu- 0 6
Annihilation in hadrons 0 7
Multiple scattering 0 10
Photoelectric 63 12
Compton scattering 0 13
Gamma conversion in e+e- 60 14
Gamma conversion in mu+mu- 0 15
Synchrotron radiation 64 23
Hadron elastic scattering 16, 17 111
Hadron inelastic 2 - 14, 18, 19 121
Neutron capture 15, 40 131
Neutron fission 0 141
Hadron stopping at rest 1, 21 - 39, 57 - 59 151
Decay 50 201
Decay with spin 0 202
Pion decay with spin 0 203
Radioactive decay 0 204
Unknown decay 0 205
External decay 0 206
Step limiter 0 401
Neutron killer 0 403
GFlash 68, 69 301

HepMC - Geant4 interface

Generator interface class takes generated events from HepMC::GenEvent and fills G4Event with the list of primary particles. To be included into Geant4 primary particle list HepMC particles should satisfy several selection criteria. HepMC particle may be considered for Geant4 tracking:

  • status code = 1
  • status code = 2 and no decay vertex is specified - status code is changed to 1
  • status code = 2 and decay vertex position has radius to the beam axis above 2.9 cm and decay products are defined
  • status code > 3 some exotica particles are tracked by Geant4 and their code status code is changed to 2 or 1 depending on existence of the predefined decay: pdgid = 17, 34, 37; 1000000 <= pdgid < 4000000
  • status code > 3 SUSY exotica does not tracked by Geant4 if charge=0 && (1000000 <= pdgid < 1000040)

The next optional check is for PDG number - the optional list of PDG numbers to be killed may be defined. By default this list keeps quarks and gluon.

After that several acceptance filters are applied for particles with status code 1:

For particles with the status code 2 and decay vertex outside radius 2.9 cm Geant4 tracks the particle and samples "forced decay". This means that predefined list of secondaries produced by the generator is assigned to the decay vertex, these secondary particles are not included into list of primary tracks for Geant4. If a decay vertex is within radius 2.9 cm the particle is not included in the list of Geant4 primary, only decay products are considered for the filter described above.

Hits

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Topic revision: r6 - 2018-05-28 - IvantchenkoV
 
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