Andreas Morsch, CERN EP/AIP CHEP 2003 Simulation in ALICE Andreas Morsch For the ALICE Offline...

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


Topics

● ALICE Simulation Strategy● Simulation in the AliRoot Framework● Virtual MC● Event Generator Interfaces


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++)


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.


Root based Event Display


Root Based Interface to G3 Geometry and Physics


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


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


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


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


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.


Interface to external generators

Event Generator Interfaces

Interface to parametrisations and decayer


Event Generator Interfaces

Container class for generators


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)


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)


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

Andreas Morsch, CERN EP/AIP CHEP 2003 Simulation in ALICE Andreas Morsch For the ALICE Offline...

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