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Andreas Morsch, CERN EP/AIP CHEP 2003
Simulation in ALICE
Andreas MorschFor the ALICE Offline Project
2003 Conference forComputing in High Energy and Nuclear Physics
Andreas Morsch, CERN EP/AIP CHEP 2003
Topics
● ALICE Simulation Strategy● Simulation in the AliRoot Framework● Virtual MC● Event Generator Interfaces
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++)
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.
Andreas Morsch, CERN EP/AIP CHEP 2003
Root based Event Display
Andreas Morsch, CERN EP/AIP CHEP 2003
Root Based Interface to G3 Geometry and Physics
Andreas Morsch, CERN EP/AIP CHEP 2003
AliRootComponents Used in Simulation
run management interface classes detector base classes data structure base classes
Detectors ITS PMDPHOSMUONTPC RICH
ZDCCRT TOF TRDFMD
STEER
EVGEN
PYTHIA
TGeant3
Geant3 MiniCERN
TGeant4
Geant4
User code for detector simulation.
User code for primary event generation
Transport MC
START
EMCAL
VZERO
HIJINGTFluka
FLUKA
External Packages
Andreas Morsch, CERN EP/AIP CHEP 2003
Virtual Monte Carlo (VMC)
User Code
VMC
Virtual Geometrical
Modeller
G3 G3 transport
G4 transportG4
FLUKA transportFLUKA
Geometrical Modeller
Reconstruction
Visualisation
See presentation by Ivana Hrivnacova
See presentation by Andrei Gheata
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
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
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 pt 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.
Andreas Morsch, CERN EP/AIP CHEP 2003
Interface to external generators
Event Generator Interfaces
Interface to parametrisations and decayer
Andreas Morsch, CERN EP/AIP CHEP 2003
Event Generator Interfaces
Container class for generators
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)
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)
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
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