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Geant4 Physics Models

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  • pg 1
									                         Geant4 Physics Models

                       Geant4 Workshop at Helsinki 30.-31.10.2003
                               T. Lampén, HIP / Helsinki




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         Outlook of Presentation
    I.          Overview of Geant4 MC simulation
    II.         Physics processes and models
    III.        Use of physics processes in practice
    IV.         Geant4 physics validation




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                                                                    HARP
 Geant4 Collaboration




                                 Univ. Barcelona               PPARC
                                                   Collaborators also from non-
                                                   member institutions, including
                                                     Budker Inst. of Physics
                                                          IHEP Protvino
                       Lebedev                          MEPHI Moscow
                                                       Pittsburg University
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     Requirements for Geant4 simulation
       ●   Geant4 : general purpose Monte Carlo toolkit for the simulation of
           the passage of particles through matter
       ●   Variety of requirements from: heavy ion physics, CP violation
           physics, cosmic ray physics, astrophysics, space science, medical
           applications, …
           => large degree of functionality and flexibility are needed
       ●   Everything open to the user
            – physics processes as well as
            – geometry
            – visualization
            – GUI
            – persistency
            – histogramming
       ●   OO methods and C++
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  Overview of Geant4 kernel                                        Geant4




                                              Visuali              Readout                    Inter
                                               zation                                          faces
Geant4 consists of 17 categories
  Independently developed and maintained                    Run                    Persis
                                                                                     tency

  Working Groups
                                                   Event
                                                                             Tracking
  Interfaces between categories maintained
  by the global architecture WG                  Digits +                    Processes
                                                    Hits
Geant4 Kernel
                                                                   Track
  Handles run, event, track, step, hit,
  trajectory                                      Geometry                    Particle


  Provides frameworks of geometrical
  representation and physics processes            Graphic
                                                    _reps
                                                                              Material

                                                                                             Intercoms

                                                                    Global



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     Tracking in Geant4
       ●   Tracking, i.e. calculating the path of particle, is the
           fundamental part in detector simulation
       ●   In Geant4, tracking is independent to particle type and
           physics processes
       ●   Tracking consists of steps
       ●   All physics processes may contribute to step length,
           interactions, generation of secondary particles, etc.
       ●   Cut in range: production thresholds for secondaries expressed
           in range instead of energy
                                             Step
                                                        End of step point
                       Begin of step point
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                                                    Boundary
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     Physics in Geant4, Basic Principles
     ●   Uniform physics models not realistic (covering wide variety of
         particle / energy range)
     ●   Instead, mixture of models used for wide (and appropriate)
         coverage of physics
     ●   Modular physics architecture also allows individual universities /
         groups to contribute their models to Geant4 and get credit
     ●   i.e. HIP has participated to implementation of INUCL and HETC
         models to Geant4
     ●   Use of polymorphism allows generic use and combination of
         different models (i.e. cross-sections and theory-driven models)


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     Physics Processes and Models
       ●   Distinguish between process and model
       ●   Each particle has to be assigned with process(es)
       ●   Processes take care of interactions and decays (also
           transportation!)
       ●   Process may consist of several
           models, cross-sections etc.
           with different energy ranges           particle

                       Gamma              process 1       process 2

                                                model 1         cross section set 1
            Compton        Pair
                                                model 2         cross section set 2
            scattering     production
                                                …               cross section set 3
                                                                …
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     Process Properties
       ●   Processes all inherit from G4VProcess
       ●   Processes do have for tracking:
              –   method GetPhysicalInteractionLength()
                  to know when interaction might happen (space-time)
              –   method DoIt()
                  to generate final state (momentum, secondaries etc.)
       ●   Processes may be:
              –   in time (decay at rest)
              –   continuously along step (Cherenkov radiation)
              –   at a point (Compton scattering)
              –   or combinations (bremsstrahlung, ionization)
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     Selecting Process in Simulation Step
          Stepping            Physics           Particle      Step        Track   Logical   Sensitive
           Manager             Process            Change                           Volume    Detector

           GetPhysicalInteractionLength
              SelectShortest
                       DoIt
                                         Fill

                                         Update
                                                 Update
                                                      IsSensitive
                                                           GenerateHits




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     Different Physics Processes and Models
       ●    7 major process categories
             – electromagnetic
             – hadronic
             – photolepton-hadron
             – decay
             – optical
             – parameterization
             – transportation
       ●    Three categories of modeling approaches:
             – theory-driven
             – parametrized
             – empirical formulae

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     In Practice

       ●   User’s responsibility to choose physics processes,
           models and cross-section tables, no default
       ●   Hadronic part most difficult (multiple models and
           cross-sections per process)
       ●   Examples and ready-made hadronic physics lists
           available ( G4 home page / site index / physics lists )
       ●   (a physics list can contain ”sub” physics lists)


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     Physics Lists - architecture




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     Examples of Available Hadronic Physics
     Lists
      •    LCG simulation project           • Low energy dosimetric
      •    HEP calorimetry                    applications
      •    HEP trackers                     • High energy production targets
      •    'Average' collider detector         e.g. 400GeV protons on C or Be
      •    Low energy dosimetric            • Medium energy production
           applications with neutrons         targets
      •    Low energy nucleon                  e.g. 15-50 GeV p on light targets
           penetration shielding
      •    Linear collider neutron fluxes   • LHC neutron fluxes
      •    High energy penetration          • Air shower applications (under
           shielding                          development)
      •    Medical and other life-saving    • Low background experiments
           neutron applications               (underground experiments)

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     Physics Lists – How-to

       ●   Class has to be derived from G4VUserPhysicsList
       ●   Methods to implement:
              –   ConstructParticle() // creates particles
              –   ConstructProcess() // assigns processes
              –   SetCuts()           // set range cuts for secondaries
       ●   Register to Geant4 run manager in main program




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     Physics Lists – example of code
       void MyPhysicsList::ConstructParticle() // take particles into use
         {
             G4Electron::ElectronDefinition();
             …
             G4AntiNeutrinoMu::AntiNeutrinoMuDefinition();
         }

       void MyPhysicsList::ConstructEM() // construct em process
         {
             …
             G4ProcessManager* pm = particle->GetProcessManager();
             if (particle->GetParticleName() == ”e-”){
                 pm->AddProcess(new G4MultipleScattering, -1, 1, 1);
                 pm->AddProcess(new G4eIonisation, -1, 2,2);
                 pm->AddProcess(new G4eBremsstrahlung, -1, -1, 3);
             }
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          }
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     Physics Lists – example of code

void MyPhysicsList::SetCuts()
  {
      defaultCutValue = 1.0 *cm;      //1.0 mm G4 recommendation
      SetCutsWithDefault();
  }

// Finally register physics list to Geant4 with this line in main program
runManager->SetUserInitialization( new MyPhysicsList );

// NOTE: also cross sections have to be registered. One can also set
   energy limits for models
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     Physics Lists – For More Information

     ●   Complete EM physics list (complete std EM) in novice
         example N03 (see and modify)
     ●   For hadronic physics lists, several models available,
         choosing right one requires care and experience
         => ready-made hadronic physics lists warmly suggested
         contains also small EM list…
     ●   Physics list derived from G4VModularPhysicsList can
         contain several lists

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     Physics Processes in Detail:
     EM Processes

       ●   Two sets of EM processes:
              –   Standard (e- binding energy ignored)
              –   Low energy (e- binding energy taken into account)
                       ●   applicable to same energy(std goes beyond 1TeV) range as
                           standard EM
                       ●   slower than standard EM, but more detailed at low energy
                       ●   uses atomic shell cross sections from DB, not parametrized
                       ●   developed for medical and space applications
                       ●   some photon and electron processes go down to 250 eV(100
                           eV from June),
                           for protons, ions and antiprotons down to about 1 keV
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     Standard EM Processes
     • Gammas                         • Muons
        – photo-electric effect          – mu ionization, energy loss
        – Compton scattering             – mu bremsstrahlung
        – e-, muon pair production       – e+e- pair production
     • Electrons                      • Charged hadrons
        – e Ionization, energy loss      – ionization, energy loss
        – e Bremsstrahlung            • All charged particles
        – e+e- -annihilation             – multiple scattering
        – sychrotron radiation           – transition radiation
                                         – scintillation
                                         – Cerenkov radiation
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     Additional Low Energy EM processes

     • Not covered in standard EM • Improved in low energy
       – Rayleigh scattering         – hadron and ion energy loss
       – Compton scattering by       – electron ionization
         linearly polarized gammas
                                     – Bremsstrahlung
       – Fluorescence
       – Auger process
                                     – photo-electric effect




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     Hadronic and Photolepton-hadron
     processes I
       ●   Hadronic processes:
              –   At rest:
                    ● Stopped muons, pions, kaons, anti-proton

                    ● Radioactive decay

              –   Elastic
                    ● One process for all long-lived hadrons

              –   Inelastic
                    ● Specific process for each hadron

                    ● Photo-nuclear

                    ● Electro-nuclear

              –   Capture
                    ● Π , K in flight
                        -   -

              –   Fission
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     Hadronic and Photolepton-hadron
     processes II
       ●   Each hadronic process may have one or more
            – cross section data sets
            – final state production models
           (with different energy ranges)
       ●   “data set” encapsulates methods and data for calculating
           total cross sections.
       ●   “model” encapsulates methods and data for calculating
           final state products
       ●   5-level architecture

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     The Decay Process

       ●   Applied to all unstable long-lived particles
       ●   Mean free path for decay λ=γβcτ,
           for most other processes λ=Nρσ/A
       ●   Same process for all particles,
           BR and decay modes from the particle’s decay table
       ●   Decay modes for heavy flavor particles not included
           in Geant4, external event generators can be used if
           needed (i.e. Pythia)

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     Optical processes
       ●   Optical photon distinguished from gamma (wavelength >>
           atomic spacing)
       ●   Optical photons are generated by:
            – Cherenkov radiation
            – Transition radiation
            – Scintillation
            – Optical processes
            – Absorption
            – Rayleigh scattering
            – Boundary processes (reflection, refraction)
       ●   Polarization taken into account, phase not (no
           interference)
       ●   Warning: no energy conservation in these processes
25     ●   Optical properties can be set for materials and volumes
T. Lampén 30.10.2003
     Shower parametrization framework

       ●   Built-in framework for shower parameterization
           available, if the ordinary tracking is not suitable
       ●   User has to concrete the parametrization process and
           assign it to a geometrical volume
       ●   Shower parameterization process can directly contact
           to sensitive detector to produce hits




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     Optional Data to Use in Geant4

       ●   G4NDL version 3.7 neutron data files (26.04 Mb)
       ●   low energy EM process data files (7.13 Mb, version
           2.2).
       ●   photon evaporation data files (7.16 Mb)
       ●   radio-active decay hadronic process data files
           (0.63 Mb)




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     Examples to Study

       ●   Good example to study physics lists and settings is
           novice example N02
       ●   N02 contains electromagnetic processes in uniform
           magnetic field
       ●   This will be studied in tomorrow’s session

       ●   http://cern.ch/geant4 > Download > examples


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     Event Biasing Techniques
     To facilitate computation, event biasing
     techniques can be used:
           –   leading particle biasing
           –   primary event biasing
           –   physics / geometry based biasing
           –   forced interaction
           –   cuts per region
           –   …

                                             Leading particle biasing –
                                        only most energetic particle followed
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     Geant4 Physics Validation
       ●   EM validation
            – thin target tests comparing Geant4 and data
              –   Comparison (not validation)Geant4 often compared to well established
                  EM simulation code EGS4
       ●   Hadronic validation
            – thin target tests comparing Geant4 and data
            – full setup HEP tests (full physics and full geometry)
       ●   Validation done extensively in large HEP experiments:
           ATLAS, CMS, BaBar, HARP
       ●   CMS just taking Geant4 into use as official simulation tool
       ●   ATLAS: Geant4 simulates relevant features of muons,
           electrons and pions usually better than Geant3
       ●   Expanded validation suites planned for users
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     User Documents at http://cern.ch/geant4
       ●   Introduction and 5 user manuals:
            – Installation guide
            – User's guide for application developers
            – User's guide for toolkit developers
            – Physics reference manual
                       ●   detailed description of each physics process with information
                           of references
              –   Software reference manual
                       ●   description of classes and methods
       ●   Examples and code
       ●   LXR source code browser
       ●   Materials of past tutorials & presentations, HyperNews and
           Web pages maintained by developers
       ●   "Geant4 general paper" - NIM A 506
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     Summary
       ●   General purpose Monte Carlo toolkit for the simulation
           of the passage of particles through matter
       ●   Particles have to be assigned with processes, which are
           responsible for physical interactions and transportation
       ●   A process can consist of several models and cross-
           sections, which have different applicability (particles,
           energy range)
       ●   Ready-made physics lists are provided especially for
           hadronic processes


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