1. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Base Classes for Simulation The ALICE Simulation Strategy Andreas Morsch ALICE Offline Group For the ALICE Collaboration 3rd LHC Computing Workshop Marseille, September 30, 1999

2. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Outline  Setting the scene Goals and Priorities AliRoot Framework  Base Class Interfaces to Simulation Components MC Base Class and Geant4 Application Status in this context Segmentation Base Class Event Generator Base Class

3. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Global Strategy  Clear distinction between immediate and long-term requirements.  Assure coherence of the whole simulation process: Event generation Particle Tracking Signal Generation Digitisation Fast simulation  Maximum (re)use of existing code and knowledge (people): Geant3 based simulation code Users come with FORTRAN+PAW+CERNLIB background

4. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Immediate Requirements  Simulations needed for Technical Design Reports Detector design optimisation Proof of principle for new physics analysis ideas Integration of new detectors  Profit from OO design as early as possible  Allow for evolution

5. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Long Term Goals  Comparison between Geant3 and Geant4 using the same geometry and data structure is mandatory (QA)  Smooth transition to Geant4 with maximum reuse of Geant3 based simulation (user-) code  Possible integration of other tracking codes (fast simulators, FLUKA?,...)  Use MC interface class to hide implementation specific features  Define G3 and G4 geometries from the same code.

6. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 AliRoot AliRoot is the ALICE off-line framework for simulation, reconstruction, and analysis. Except for Geant3 and some remaining legacy code, this framework is based on OO design and written in C++. It uses the ROOT system as a foundation.  Framework helps people to move into OO CINT C++ scripting language persistent=transient object …  Keeps entropy low and allows to speak a common language.

7. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999

8. ROOT provides I/O (transient=persistent) C++ Scripting Language Container Classes Automatic Documentation GUI Classes Graphic Tools Histograming Tools ….

9. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Base Classes Representing Simulation Components Physics Signals Fast Simulation Detector Simulation Hits o o Digits o o Reconstruction Physics Performance Feasibility Studies Detector Performance Reconstruction Optimisation Event-Generators Detector ClassesSegmentationMCParticle Stack Example: Fast Simulation Example: Digitisation Cluster Finder Tracking Codes

10. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 MC Base Class and Run Control  AliMC interface class Geometry definition methods Material and Tracking Media Definitions Physics Process Control Generic access functions to information during tracking  AliRun Run Control Event Control Owner of … Particle Stack Detector list Primary particle Generator Magnetic Field Map I/O control: kinematics tree hits digits reconstruction objects Instantiation during run time using CINT: new TGeant3(“C++ Interface to Geant3”); TGeant3 *geant3 = (TGeant3*) gMC; new TGeant4(“C++ Interface to Geant4”); TGeant4 *geant4 = (TGeant4*) gMC;

11. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 New Tracking Schema Detector Version StepManager Add the hit GUSTEP FLUKA Step Geant4 StepManager Disk I/O Objy Root AliRun::StepManager

12. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 AliMC Implementations  TGeant3up and running  TGeant4 in development  TFlukaon the wishlist

13. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Geant3->Geant4  Starting Idea Reuse the Geant4 g3tog4 package for implementation of geometry methods of the MC interface.  g3tog4 completely re-implemented bugs fixed missing parts have been added reflection symmetry still missing in G4  ROOT output of hits for direct comparison G3/G4.  No results from tracking yet.

14. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 ALICE G4 Simulation Prototype Components Stand-Alone Seen through AliMC

15. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 More examples for the use interface classes in AliRoot  Segmentation of Muon Chambers (CPC+CSC) detector response simulation digitisation clustering visualisation  Event generators slow simulation fast simulation

16. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Alice Muon Arm Segmentation base class was developed out of the need to simulate CPCs and CsC with segmentation schemas changing from chamber to chamber, radially … and with time.

17. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 ALICE Muon-Arm Class Design AliMUONsegmentation AliMUONchamber TObject AliMUONresponse AliDetector AliMUON Segmentation Behaviour: Local to global Transformation Global to local Transformation Iterators Next Neighbours Generic access functions to geometrical structure …. Reflects the physical structure of the detector...

18. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Clients of the Segmentation Class AliMUONresponse AliMUONsegmentation AliMUONHitMap Stepmanager Signal Generation Condition Cluster Generation Digitisation Rawcluster Finder Space Point Reconstruction Hits Pad Hits Digits RawClusters SpacePoints Simulation and Reconstruction functional and ready for performance tests Event DisplayAnalysis

19. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999

20. Hits and Pad Hits

21. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Hits and PadHits

22. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Hit Reconstruction

23. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Event Generator Base Class  Purpose: Generate particles to be tracked and put them on the stack.  Interaction with the gAlice global object: gAlice->SetGenerator(this) registers the generator object to be used for the run. The generator created first is used. gAlice->SetTrack(...) puts particle in the stack.  In a more general framework AliGenerator can be used to write primary particle event files or as input to fast physics simulation.

24. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 AliGenerator Member Functions  Member Functions... Generation (Init(), Generate()) Set and get number of particles/event Set kinematic selection (momentum, pT, phi, theta, y) Set vertex position and smearing (sigma, per event, per track) Set child and parent weight

25. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Event Generator Implementations  External Generators (Pythia)  External Event Files  Parameterisations (y, pT, particle cocktail)  Boundary Sources as interface to FLUKA  Testing Tools: Particle Guns,...

26. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Interface to FLUKA: AliGenFLUKASource FLUKA AliRoot ALIFE Boundary Source

27. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Application: AliGenCocktail: AliGenCocktail: public AliGenerator Tlist AliGenCocktailEntry Generator Events Generated first/last in stack biasing weight AliGenerator A fast physics cocktail simulation tool:

28. Andreas Morsch CERN/ALICE 3rd LHC Computing WorkshopMarseille, September 30, 1999 Summary  ALICE uses ROOT based OO frame-work for simulation and reconstruction (AliRoot)  AliMC abstract interface class allows: Integration of Geant3 based simulation code Test of Geant4 under the same conditions (Geometry, signal generation, output data structures)  Base classes representing simulation related concepts produce synergy effects inside our framework.  … and could be reused in other architectures