Geant4: an update

1. Geant4: an update An overview of Geant4’s recent developments John Apostolakis, CERN Makoto Asai, SLAC for the Geant4collaboration

2. Outline • Brief introduction to Geant4 • Physics highlights • Modeling • validation • New capabilities • Detector description and collision detection • Some current Developments • In progress • Planned for 2003

3. Introduction Part 1 Context Toolkit structure

4. GEANT 4 introduction • Detector simulation tool-kit for HEP • offering alternatives, allowing for tailoring • Software Engineering and OO technology • provide the method for building, maintaining it. • Requirements from HEP & other domains: • LHC, heavy ions, CP violation, cosmic rays • medical and space science applications • World-wide collaboration • RD44 1994-1998 • MoU 1999-today

5. Geant4 Overview • Extensive & transparent physics models • electromagnetic, hadronic, optical, decay, … • Powerful structure and kernel • tracking, stacks, geometry, hits, … • Interfaces • visualization, GUI, persistency. • Efficiency enhancing techniques • Framework for fast simulation (shower parameterization) • Variance reduction / event biasing

6. Physics Highlights Part 2 Modeling and Validation

7. Physics Development Highlights Geant4 releases Dec 2001-today included • New EM processes • And improvements to existing processes • New theoretical hadronic models • In particular for the cascade energy range • The release of ‘tailored’ physics lists • For different hadronics use cases. • Numerous physics improvements Including, for example • Charge state for recoils • Improved X-sections for e-Nuclear, with hard scattering

8. Significant developments in EM (std) in 2002 • Multiple scattering (L. Urban) • Angular distributions (see next slides) • Ultra relativistic energies (H. Burkardt, S. Kelner, R. Kokoulin) • gm m process • Ionization for Generic Ions (V. Ivanchenko) • New model of Transition radiation (V. Grichine) • for TR detectors • Redesign of few processes • prototype model approach for Ionization and Bremsstrahlung (V. Ivanchenko)

9. Multiple scattering • 15.7 MeV electrons on gold foil Small differences between G4 & G3 observed below 1 MeV • Results competitive versus data in G4 3.2 • Differences traced to Multiple Scattering MS modeling improved in Geant4 4.0 & 5.0 Examples of comparisons to data Thanks to L. Urban Geant4 4.0 (Dec 2001) Angle (deg)

10. Multiple scattering • Refined modeling of angular distributions • in Geant4 5.0 Modeling & comparisons: L. Urban Geant4 5.0 (Dec 2002) Angle (deg)

11. Element particle Energy CHIPS QGSM Hadronic physics: models, processes and ‘lists’ Components can be assembled in an optimized way for each use case. • Five level implementation framework • Variety of models and cross-sections • for each energy regime, particle type, material • alternatives with different strengths and CPU requirements. • Illustrative example of assembling models into an inelastic process for set of particles • Uses levels 1 & 2 of framework Pre-compound model Parame- terized

12. Models: Cascade energy range • Parameterized process (1997) • Chiral Invariant Phase Space decay,“CHIPS” • For g-Nucleus, p capture, string-’backend’ • First release Dec 2001 in Geant4 4.0 • Refinements and extension in 2002 • Bertini cascade (Dec 2002, Geant4 5.0) • Re-engineered from HETC by HIP • See the presentation of A Heikinen • Binary cascade model (Frankfurt, CERN) • First release for nucleon induced interactions (in G4 5.0) • Extensive verification suite • See the presentation by D. Wright, V. Ivantchenko • For further details, • see the next presentation (J.P. Wellisch) M Kosov, P Degtyarenko, JP Wellisch A Heikinen N Stepanov JPW G Folger JPW

13. Tailored Physics ‘lists’ • Created and distribute “educated guess” physics lists • That correspond to the major use cases of Geant4 involving hadronic physics, • to use directly, and as a starting point for users to modify, • facilitate the specialization of those parts of hadronic physics lists that vary between use cases. • First released in September 2002 • Using physics models of Geant4 4.1. • Revised with experience of comparisons with data • Latest: • updated with physics models of Geant4 5.0 in March 2003 • Find them on the G4 hadronic physics web pages http://cmsdoc.cern.ch/~hpw/GHAD/HomePage

14. HEP calorimetry. HEP trackers. 'Average' HEP collider detector Low energy dosimetric applicationswith neutrons low energy nucleon penetration shielding linear collider neutron fluxes high energy penetration shielding medical and other life-saving neutron applications low energy dosimetric applications high energy production targets e.g. 400GeV protons on C or Be medium energy production targets e.g. 15-50 GeV p on light targets LHC neutron fluxes Air shower applications low background experiments Use cases of Physics Lists Contributors: http://cern.ch/geant4/organisation/ working_groups.html#wg.Had

15. LHEP is the fastest for CPU uses the LEP and HEP parameterized models for inelastic scattering. QGSP, uses theory-driven modeling for reactions of ps, Ks, and nucleons. It employs Quark Gluon String Model for the 'punch-through' interactions of the projectile A Pre-equilibrium decay model with an extensive evaporation phase to model the nucleus 'after the punch'. QGSC, is similar but uses CHIPS for fragmentation The CHiral Invariant Phase-Space decay (CHIPS) FTFP replaces instead the string with a diffractive string excitation similar to that in FRITJOF, and the Lund fragmentation functions. Physics lists for calorimetry

16. Comparison projects • Joint efforts for comparing Geant4 with experiment & test-beam data. • Results of EM comparisons: ‘peak’ between 2000-2002. • Hadronic comparisons: 2002-ongoing. • Collaboration with experiments • ATLAS (projects with data of numerous test beams) • BaBar (with data for tracker, drift chamber) • Many results have been presented at conferences & workshops, eg Calor 2002. • And at regular LHC experiment-Geant4 physics comparisons meetings

17. Linearity Mix and match problem seen in parameterised models. Problem disappears, as expected, when utilising theoretical models: Eg Quark-Gluon String Model + CHIPS For latest results please see the presentations of JP Wellisch & Atlas First results from April/May 2002 Thanks to Atlas HEC and J.P. Wellisch

18. As presented at Geant4 Workshop, 30th September 2002 Thanks to P. Loch, Atlas Geant4 Hadronic Signals in ATLAS Calorimeters • Calorimeter pion response: • after discovery of “mix-and-match” problem (transition from low energy to high energy char-ged pion models) in the deposited energy from energy loss of charged particles in pion showers in the HEC (G4 4.0, early 2002): fixes suggested by H.P. Wellisch (LHEP, new energy thresholds in model transition + code changes) and QGS model tested; • e/π signal ration in HEC and TileCal still not well reproduced by Geant4 QGS or LHEP - but better than with GCalor in Geant3.21; • energy dependence in HEC in QGS smoother, “discontinuities” between ~20 GeV and ~80 GeV gone; e/π signal ratio HEC Pions QGS LHEP e/π signal ratio Pion energy [GeV] Word highlights: JA, March 2003

19. New capabilities Part 3 Detector description Performance Visualisation

20. Other Development highlights • Detector description • New ways to create geometries • Tools to detect incorrect geometry definitions • A different field for any volume (or volume tree) • Overriding a global field • Ability to reduce initialisation time • By saving/retrieving physics processes’ table • Variance reduction / event biasing • Importance: biasing by geometry • Leading particle biasing

21. Improvements in Geometry • Reflection of volume hierarchies • Eg to create endcap geometry • Improved voxelisation for performant navigation • 3-D for parameterized volumes • Now equal performance to ‘placed’ volume • Option to avoid voxelizing some volumes • ‘Illegal’ geometries detected & rejected • E.g. incompatible daughters (placed & parameterized) • XML binding: GDML 1.0 released • Specification & Implementation • Refinements currently on ‘hold’. I Hrivnacova G Cosmo V Grichine G Cosmo G Cosmo R Chytracek

22. Debugging geometries • It is easy to create overlapping volumes • a volume that protrudes from its mother, • 2+ volumes that intersect in common mother • During tracking Geant4 does not check for malformed geometries • The problem of detecting ‘significant’overlaps is now addressed by • DAVID that intersects volumes directly ( Uses graphical representations ) • Created by S. Tanaka, released ca 1997 • New commands to run verification tests • Created by DC Williams; released in 4.0 • New example with full tracking / navigation • Created by M Liendl; released in 5.0 Thanks to S. Tanaka

23. Variance reduction • Geant4 had leading particle biasing option for “low energy” neutrons. • Now redesigned and improved, implementation in Geant4 4.1. • It was possible to use other methods, but only in user code. • Now new general purpose built-in methods have been released • Further refinements & methods are under development. • Importance biasing: • Splitting/Russian roulette (first released in G4 4.1, June 2002). • Importance values can be associated to a volume • In the ‘mass’ geometry or in a dedicated ‘parallel’ geometry. • Enabling simulation of shielding applications with improved time efficiency by large factors • Varied options in driving MC ‘history’ and scoring tallies • No changes to the kernel were required, due to the flexibility of the toolkit. • Leading particle biasing • a-la MARS 95, for En<5GeV M Dressel N.Kanaya

24. CPU Performance • Our first simple benchmarks: • Geometry faster, EM shower setups: competitive • Performance in experimental setups (with Geant4 releases 2 and 3) was comparable to Geant3 • few counterexamples, including BTeV ECAL. • New performance issues arose with Geant4 4.0 • and were addressed (in the patches & release 4.1) • Difficult cases remain, including • Some setups of EM showers and field propagation, factor ~ 2x • Collecting a set of benchmarks • To follow computing performance regularly • Goal is that Geant4 is at least as fast as Geant3 in almost all cases • When its power is used.

25. DAWN renderer Thanks to S. Tanaka Visualization Geometry, hits • New • “DTREE”: hierarchy display • HEPREP driver for WIRED • Other Current Drivers • OpenGL • VRML • DAWNRenderer • Also from others, eg • IGUANA (for CMS simulation) Iguana, thanks to L.Tuura, I. Osborne

26. Current development highlights Part 4 Imminent Scheduled

27. In Progress 2003 (highlights) • Cuts per region • See next slides • Improvements of multiple scattering • in straggling, backscattering • Additional refinements of physics lists • Continuous updates • Design iteration of EM (std) processes • With benefits in tailoring, maintenance • Further extension and automation of testing • Statistical testing: ‘benchmarks’ and test-beams

28. Cuts in Geant4 (to date) • Geant4 has had a unique production threshold (‘cut’) expressed in length (range of secondary). • For all volumes • Possibly different for each particle. • This promotes • Clear criteria for locality of energy deposition • better use of CPU – less ‘wasted’ in dense materials • Yet appropriate length scales can vary greatly between different areas of a large detector • Eg a vertex detector (5 mm) and a muon detector (2.5 cm). • Having a unique (low) cut can create a performance penalty. • So the part of the detector with the lowest cut need fixed the cut for all the simulation.

29. Motivation for several cuts • Having a unique cut enforced a choice between • Sacrificing accuracy of energy deposition • Accepting a performance penalty • Lifting the uniqueness of cuts • Requested from LHC experiments & BaBar • Implemented by introducing geometrical ‘regions’ • And enabling the choice of thresholds in a region.

30. Region & its properties • Introduce the concept of « region »: • Set of geometry volumes, typically of a sub-system; • Eg: barrel + end-caps of the calorimeter; • Or any group of volumes; • A cut in range is associated to a region; • a different range cut for each particle is allowed in a region . • Typical Uses • barrel + end-caps of the calorimeter can be a region; • “Deep” areas of support structures can be a region. Region B Region B Region B Region C c Region B Region A

31. Cuts per region status • Design and implementation have been made • without severe design revision of the existing GEANT4; • First implementation available in latest b release (Feb) • Comparable run-time performance • Today a penalty within 5% is seen, due to redundant checks included for verification purposes • ‘Full release’ will be in Geant4 5.1 (end April) • With further refinements, tests, validation.

32. MS in progress Multiple scattering: Refinements • Backscattering • Straggling • Transmitted energy

33. MS: straggling Fit to data Lateral straggling of 2.5 MeV protons After mylar foils Geant4 5.1 (April 2003)

34. In progress (also) • The refinement of the design of EM physics processes through the use of ‘models’. • To enable the specialization of key features; • To enable the easy use of different models for a single process (e.g. Ionization) in one application. • Additional variance reduction techniques • Filter for enhancing processes in hadronic interactions.

35. Some further 2003 development highlights • Additions to physics processes/models • p induced binary cascade model, .. • EM-std implementation with “model” approach. • Refinements, including • Improvement to recoil in elastic scattering • Improved X-sections for pions. • Revisions of the ‘tailored’ physics lists • Incorporating results of validation • Variance reduction • Physics process enhancement • Leading particle biasing • Plus refinements to importance biasing

36. Review and Releases • d Review October 2002 • Report available at http://cern.ch/geant4 • Developments available in b releases • Every two months • Latest b release (February) • Included cuts per region • Upcoming releases • Next minor release is Geant4 5.1 planned for end-April • Incorporating cut per region, developments in progress. • Release timeframe selected to aid in CMS production. • ‘Scheduled’ release Geant4 5.2 for end-June • Further refinements, developments • 2003 work items & planned release contents to be available soon • Started from User & Experiment Requirements and Requests • Next major release Geant4 6.0 is scheduled for December 2003.

37. http://cern.ch/geant4/ Summary • Results of comparing Geant4 versus data, • Have & are providing excellent ‘yardsticks’ of EM perf. • Are testing the hadronics well, with increasing coverage • Geant4 has demonstrated important strengths: • stability of results, flexibility, transparency. • it is in production use today in running HEP experiments (BaBar, HARP) • Geant4 is evolving • With the feedback from LHC exper., BaBar and numerous other experiments and application domains. • Refinements & development are ongoing.

38. THE END Thanks to all Contributors Users

39. After the END … Slides after this are backups, not part of the presentation. v0.8 24th March 2003, 18:40 GMT

40. Electromagnetic physics • Gammas: • Gamma-conversion, Compton scattering, Photo-electric effect • Leptons(e, m), charged hadrons, ions • Energy loss (Ionisation, Bremstrahlung) or PAI model energy loss, Multiple scattering, Transition radiation, Synchrotron radiation, • Photons: • Cerenkov, Rayleigh, Reflection, Refraction, Absorption, Scintillation • High energy m • Alternative implementation • ‘Standard’ for applications that do not need to go below 1 KeV • ‘Low Energy’: down to 250eV (e+/g), O(0.1) mm for hadrons • Including specialized HEP applications

41. Shower profile 1 GeV electron in H2O G4, Data G3

42. Support: new & continued • Documentation • Revisions of the user and reference guides • After assessments of overall structure & detailed • LXR for code reference • see http://geant4www.triumf.ca/lxr/ • New tool for collecting requirements • Continued Support • of users’ questions, problems • HyperNews, Problem reporting system, email. • of comparisons with data • By wide variety of users, in HEP, space, medical phys., ..

43. Testing and QA 2002/3 • Establishment of ‘statistical testing’ suite • Automated comparison of physics quantities • Against ‘standard’ data (eg NIST) • In ‘test-beam’ applications • Including ‘regression testing’. • For details see • Establishing a benchmark suite for computing performance.

44. Examples of improvements Fixes and improvements in Geant4 release 4.1 (June 2002) • Geometry • Fix for voxelisation of reflected volumes • Fix for exit normal angle • Fix for problem in very small step in field • EM • Improvements in Multiple Scattering, Ionisation, .. • Hadronics • Fix for energy conservation in parametrised models. • Fix for small peak at f=0 in parametrised models.

45. BaBar Geant4 based simulation since 2001 production. More than 109 events (through Oct 2002) Used Geant4 3.1+fixes, own transport.

46. Cuts/Region Introduction • A Cut here is a « production threshold »; • Only for physics processes that have infra-red divergence • Not tracking cut; (which does not exist in Geant4) • GEANT4 up to now allows a unique cut in range; • One cut in range for each particle; • By default is the same cut for all particles; • Consistency of the physics simulated: • A volume with dense material will not «dominate» the simulation time at the expense of sensitive volumes with light material. • Requests from ATLAS, BABAR, CMS, LHCb, …, to allow several cuts; • Globally or per particle;

47. Geant4 Collaboration Collaborators also from non-member institutions, including Budker Inst. of Physics IHEP Protvino MEPHI Moscow Lebedev