1. Andreas Morsch, CERN EP/AIP CHEP 2003 Simulation in ALICE Andreas Morsch For the ALICE Offline Project 2003 Conference for Computing in High Energy and Nuclear Physics

2. Andreas Morsch, CERN EP/AIP CHEP 2003 Topics ● ALICE Simulation Strategy ● Simulation in the AliRoot Framework ● Virtual MC ● Event Generator Interfaces

3. Andreas Morsch, CERN EP/AIP CHEP 2003 ALICE Simulation Strategy (1) ● Coherent simulation framework for detector and physics performance studies in the AliRoot Framework based on ROOT – Physics simulation – Detailed detector response simulation – Fast simulation ● Use of Transport MC transparent to the user – Virtual MC Interface ● Maximum reuse of user code ● One single development line (in C++)

4. Andreas Morsch, CERN EP/AIP CHEP 2003 ALICE Simulation Strategy (2) ● Event Generator Interface tailored to the needs of the Heavy Ion Physics Community – Soft Uncorrelated Background ⊕ Correlations ⊕ Hard Processes – Maximum of flexibility and user configuration capabilities ● Monte Carlo Truth – Full history from primary particles (partons) to hits available after simulation.

5. Andreas Morsch, CERN EP/AIP CHEP 2003 Root based Event Display

6. Andreas Morsch, CERN EP/AIP CHEP 2003 Root Based Interface to G3 Geometry and Physics

7. Andreas Morsch, CERN EP/AIP CHEP 2003 AliRoot Components Used in Simulation run management interface classes detector base classes data structure base classes Detectors ITSPMDPHOSMUONTPCRICH ZDCCRTTOFTRDFMD STEER EVGEN PYTHIA TGeant3 Geant3MiniCERN TGeant4 Geant4 User code for detector simulation. User code for primary event generation Transport MC START EMCAL VZERO HIJING TFluka FLUKA External Packages

8. Andreas Morsch, CERN EP/AIP CHEP 2003 Virtual Monte Carlo (VMC) User Code VMC Virtual Geometrical Modeller G3 G3 transport G4 transport G4 FLUKA transport FLUKA Geometrical Modeller Reconstruction Visualisation See presentation by Ivana Hrivnacova See presentation by Andrei Gheata

9. Andreas Morsch, CERN EP/AIP CHEP 2003 Virtual Monte Carlo Status ● TGeant3 – Used in production ● TGeant4 – Used for Geant4 physics validation – See talk by Isidro Gonzalez ● TFluka – Under development – Full chain from primary particle generation, transport, hits running – See also talk by Alberto Fasso on FLUKA physics

10. Andreas Morsch, CERN EP/AIP CHEP 2003 Simulation of Heavy Ion Collisions ● Shortcomings of existing generators – None of the existing generators do give detailed account of the expected multiplicities, pt and rapidity dependence at LHC energies – Most of the hard probes (heavy flavor, jets...) are not properly reproduced by existing generators. – Existing generators do not provide for event topologies like momentum correlations, azimuthal flow etc. ● The small cross-section of hard processes would demand prohibitively long runs to simulate a number of events that is commensurable with the expected number of detected events in the experiment

11. Andreas Morsch, CERN EP/AIP CHEP 2003 The ALICE Approach ● The simulation framework provides an interface to external generators, like HIJING. ● A parameterised “signal free” underlying event with multiplicity as a parameter is provided. ● Rare signals can be generated using – External generators like PYTHIA – Libraries of parameterised p t and rapidity distributions ● The framework provides a tool to assemble events from different signal generators – On the primary particle level (cocktail) – On the digit level (merging) ● After-Burners are used to introduce particle correlations.

12. Andreas Morsch, CERN EP/AIP CHEP 2003 Interface to external generators Event Generator Interfaces Interface to parametrisations and decayer

13. Andreas Morsch, CERN EP/AIP CHEP 2003 Event Generator Interfaces Container class for generators

14. Andreas Morsch, CERN EP/AIP CHEP 2003 Afterburner Processors ● Introduction of correlations in otherwise uncorrelated events – 2 particle correlations – Flow – Assembling of new events ● Design of classes involved in event generation (AliRun, AliStack, AliGenerator) supports requirements for Afterburner – Example: Several objects of type AliStack containing the input events can be connected to the Afterburner (of type AliGenerator) to fill a stack connected to AliRun (output event)

15. Andreas Morsch, CERN EP/AIP CHEP 2003 Fast Simulation ● As many approaches as subdetectors. ● Two main applications in ALICE: – Very fast simulation ● Smearing of track parameters from paramterisation obtained from slow simulation – Combined slow and fast simulation ● Detailed simulation of inner detector system (vertexing) and fast simulation of TPC response (momentum resolution)

16. Andreas Morsch, CERN EP/AIP CHEP 2003 Conclusions ● The ALICE Offline Project has developed a coherent simulation framework for detector and physics performance studies in the AliRoot Framework based on ROOT ● The main simulation components are – Virtual MC Interface ● TGeant3 (production) ● TGeant4 (physics validation) ● TFluka (implementation ongoing) – Event generator interfaces tailored to the needs of the heavy ion community