1. Guided Tour of Geant4 Physics Lists II Geant4 Short Course at JPL 10 November 2006 Dennis Wright

2. 1 Outline Introduction Using the process/model catalogue Reference physics lists A physics list for the space users community

3. 1 Why Do We Need a Physics List? ● Physics is physics – shouldn't Geant4 provide, as a default, a complete set of physics that everyone can use? ● No: there are many different physics models and approximations very much the case for hadronic physics – a field which is still developing but also the case for electromagnetic physics computation speed is an issue a user may want a less-detailed, but faster approximation no application requires all the physics and particles Geant4 has to offer e.g., most medical applications do not want multi-GeV physics

4. 1 Why Do We Need a Physics List? ● For this reason Geant4 takes an atomistic, rather than an integral approach to physics provide many physics components (processes) which are de-coupled from one another user selects these components in custom-designed physics lists in much the same way as a detector geometry is built ● Exceptions: a few electromagnetic processes must be used together future processes involving interference of electromagnetic and strong interactions may require coupling as well

5. 1 particle at rest process 1 in-flight process 2 process 3 process n model 1 model 2. model n c.s. set 1 c.s. set 2. c.s. set n Cross section data store Energy range manager

6. 1 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV LEP HEP ( up to 15 TeV) Photon Evap Multifragment Fermi breakup Fission Evaporation Pre- compound Bertini cascade Binary cascade QG String (up to 100 TeV) FTF String (up to 20 TeV) High precision neutron At rest Absorption  K, anti-p Photo-nuclear, electro-nuclear CHIPS (gamma) CHIPS Hadronic Model Inventory LE pp, pn Rad. Decay

7. 1 Many Physics Lists to Choose From Advanced examples see talk by M.G. Pia Novice examples physics lists range from simple to “real world” N03 has a complete, standard EM physics list for many applications just copy and use also very educational Reference physics lists ready-made for specific applications Building your own using the process/model catalogue

8. 1 Process/Model Catalogue (1) Geant4 processes and models were not sufficiently documented there is a Physics Reference Manual, but by design this does not tell users how to invoke physics the Application Developers Guide discusses physics lists but does not give physics advice how is a user to know which processes and models to use? Process/model catalogue is first step towards filling the gap Additional tools to come: executive summaries of physics lists

9. 1 Process/Model Catalogue (2) Catalogue provides: hyper-linked list of particles in Geant4 for each particle a list of processes which may be assigned for each process a list of models and cross sections which may be assigned short explanation of each process or model link to physics list documentation Can be found at Geant4 web page ->user support->process/model catalogue geant4.web.cern.ch/geant4/support/proc_mod_catalog

10. 1 Process/Model Catalogue (3) Demonstration

11. 1 Reference Physics Lists (1) ● Because of user requests, a set of ready-made physics lists was prepared for specific applications ● Documentation can be found on the Geant4 web page at http://geant4.web.cern.ch/geant4/physics_lists ● Caveats: these lists are provided as a “ best guess ” of the physics needed in a given case The user is responsible for validating the physics for his own application and adding (or subtracting) the appropriate physics “Trust, but verify.” they are intended as starting points or templates

12. 1 Reference Physics Lists (2) ● Code in geant4/physics_lists (parallel to source directory) ● Build physics list libraries: go to geant4/physics_lists/hadronic set G4INSTALL to point to Geant4 source libraries make in your main() instantiate the desired physics list and pass pointer to Run Manager ● Sophisticated design maximizes re-use of code uses templates very easy to change physics lists in a given application drawback: difficult to understand what models, processes are included

13. 1 Reference Physics Lists (LHEP) ● Standard EM physics processes ● Low Energy and High Energy Parametrized (LEP, HEP) models for all hadrons LEP and HEP models are the re-engineered versions of the GHEISHA models (parametrized from data) fast energy is conserved on average, not event-by-event Pros and cons: good shower shape not so good for detailed reactions one of the most validated physics lists ● Used as the “backbone” for all other physics lists insert better models where available for given particles, energies

14. 1 Reference Physics Lists (QGSP) ● Standard EM physics processes ● Quark-gluon string model for high energies (> 20 GeV) theoretically motivated – good predictive power pre-compound model handles nuclear de-excitation ● Low Energy Parametrized (LEP) for low energies (< 20 GeV) ● Hyperons, anti-baryons use LEP, HEP (all energies) ● Gamma-nuclear model added for E < 3.5 GeV ● Pros and cons: designed for use in HEP calorimeters and trackers one of the most validated physics lists some problems with shower shapes at high energies (starts too early, ends too early)

15. 1 Reference Physics Lists (QGSP_BERT) ● Standard EM physics processes ● Quark-gluon string model for high energies (> 20 GeV) ● Low Energy Parametrized (LEP) for medium energies (10 < E < 20 GeV) ● Bertini-style cascade for low energies (< 10 GeV) classical cascade model, uses free-space cross sections ● LEP, HEP for all anti-baryons ● Gamma-nuclear model added for E < 3.5 GeV ● Pros and cons: designed for use in HEP trackers, collider detectors good for neutrino beams, kaon interactions mostly OK for cosmic ray applications (except for heavy ions)

16. 1 Reference Physics Lists (QGSP_BIC) ● Standard EM physics processes ● Quark-gluon string model for high energies (> 20 GeV) ● Low Energy Parametrized (LEP) for medium energies (3 < E < 20 GeV) ● Binary cascade for low energies (< 3 GeV) detailed theory-driven model upper limit due to dependence on resonances ● LEP, HEP for hyperons, anti-baryons, LE kaons ● Gamma-nuclear model added for E < 3.5 GeV ● Pros and cons: designed for use in HEP trackers important energy region for calorimetry only covered by LEP model

17. 1 Reference Physics Lists (QGSP_BERT_HP) ● Same as QGSP_BERT except: High precision neutron package used for E n < 20 MeV ● Requires G4 Neutron Data Library (G4NDL) ● Pros and cons: designed for shielding studies, neutron flux studies handles down to thermal neutrons can be slow if there are lots of neutrons

18. 1 Reference Physics Lists (LBE) ● Standard EM physics processes for e +     ● Low Energy EM processes for  e -, ions, hadrons ● Also optical photons, scintillation, etc. ● Radioactive decay ● LEP, HEP models for hadronic interactions ● High precision neutron models used for E n < 20 MeV ● Pros and cons: designed for low-background experiments (e.g. dark matter searches) “flatter” code design: easier to read/understand

19. 1 A Physics List for the Space Users Community (1) ● Developed in consultation with Robert Weller, Marcus Mendenhall (Vanderbilt) very similar to QGSP_BERT better treatment of ion-ion physics “flat” code design for better transparency available from SLAC Geant4 website: ww.slac.stanford.edu/comp/physics/geant4/ slac_physics_lists/micro/space_elect_physics_list.html Meant to be used as a template, or starting point Hadronic ion physics limited to ~10 GeV/N Could also be used for cosmic ray applications

20. 1 A Physics List for the Space Users Community (2) ● Demonstration