ALICE Simulation Framework

Ivana Hrivnacova1 and Andreas Morsch2

1NPI ASCR, Rez, Czech Republic

2CERN, Geneva, Switzerland

For the ALICE Collaboration

International Conference on

COMPUTING IN HIGH ENERGY AND NUCLEAR PHYSICS

Padova, February 9, 2000

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Outline

Setting the scene goals and priorities AliRoot framework

Interfaces to simulation components Monte Carlo

Geant4 application status in this context event generator segmentation

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Strategy

Distinction between immediate and long-term requirements.

Assure coherence of the whole simulation process: event generation particle tracking signal generation digitization fast simulation

Reuse of existing code and knowledge (people): Geant3 based simulation code users come with FORTRAN+PAW+CERNLIB background

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StrategyShort And Long Term

Goals Short term requirements:

simulations are needed for: Technical Design Reports, detector design optimization, integration

of new detectors profit from OO design as early as possible allow for evolution

Long term goals: smooth transition to Geant4 reuse of Geant3 based simulation user code: possible integration of other tracking codes

fast simulators, FLUKA, ...

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AliRoot

AliRoot = the ALICE off-line framework for simulation, reconstruction, and analysis OO design, C++ Geant3 and some legacy code in FORTRAN

Based on ROOT framework

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run management interface classes detector base classes data structure base classes

AliRootComponents Used in

SimulationDetectors ITS PMDPHOSMUONTPC RICH

ZDCCASTOR TOF TRDFMD

STEER

EVGEN

PYTHIA

TGeant3

Geant3 MiniCERN

g4mc aliceG4

Geant4

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Monte Carlo Interface

Pure abstract class AliMC It has been developed as generalization of G3 functions for

definition of simulation task Provides methods for

geometry description definition physics process control access functions to tracking particle properties visualization

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Monte Carlo Interface Implementations

For Geant3 = TGeant3 class up and running

For Geant4 = g4mc package in development each domain is covered by its manager class:

geometry, physics, stepping, visualization, run each manager uses corresponding category(ies) of G4

For FLUKA = no implementation yet on the wish list

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Monte Carlo Interface Implementation for

Geant4

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MC Implementation for G4

Geometry (1) Geometry manager as client of g3tog4 (stand-alone

package provided by Geant4 for automatic conversion of G3 geometry)

This development resulted to our contribution to g3tog4 in Geant4

In difference from standard usage of g3tog4 the input geometry is not the ZEBRA file (converted to ASCII file) but the C++ code in detector classes in AliRoot for debugging reasons the ASCII file can be generated from

AliRoot, read back and process by standard g3tog4 tool, too

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MC Implementation for G4

Geometry (2) Almost all G3 options for geometry definition are

supported passing parameters from mother volume to its daughters divided volumes - represented by replicated physical volumes in

G4 (G4PVReplica)

Unsupported option: “MANY” “MANY” option substitutes lack of Boolean operations in G3

geometry

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MC Implementation for G4

Physics Physics manager provides G4 physics list construction

from G3 cuts and physics process control parameters G3 tracking media parameters are applied to G4 logical

volumes with usage of user limits (derived class from G4UserLimits) special cuts process (derived class from G4VProcess) special flags (process control) process (derived class from

G4VProcess)

In development only subset of G3 parameters is supported more testing needed

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MC Implementation for G4

Stepping, Visualization, Run

Stepping step manager class works as adapter between the MC interface

(AliMC) and G4 step manager (G4StepManager) access to properties of the tracking particle during stepping complete

Visualization visualization manager class is designed to adapt the MC interface

methods to G4 visualization work has been started recently

Run Management run manager class provides G4 run control to the application main

program or its manager (AliRun in AliRoot)

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Monte Carlo Interface

ALICE Geant4 Geometry

Detectors: TPC, RICH, FMD, CASTOR, MUON, PHOS, PMD, ZDCStructures: HALL, ABSO, DIPO, FRAME, MAG, PIPE, SHIL

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Event Generator Interface

Class AliGenerator Purpose: to generate primary particles to be tracked and

to put them on the stack Functions:

make generator known to the run manager (AliRun) set kinematic selection (momentum, pT, phi, theta, y) set vertex position and smearing (sigma, per event, per track) set child particle and parent particle weight

It can be also used to write primary particle event files as input to fast physics simulation

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Event Generator Interface

Implementations External generators: Pythia External event files Parameterizations (y, pT,

particle cocktail) Boundary sources as interface

to FLUKA Testing tools: particle guns, …

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Event Generator Interface

Generator Cocktail Recursive implementation of AliGenerator Enables to compose event from more different generators

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Event Generator Interface

Interface to FLUKA

FLUKA

AliRoot

ALIFE Boundary Source

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Segmentation Interface

Class AliMUONSegmentation Common “behavior” of detector segmentation:

pad to real coordinate transformation iteration over pads providing pad neighbors access functions to geometry

Segmentation of Muon Chambers are used in signal generation (spreading charge over pads) recursive cluster finding hit reconstruction from clusters visualization of hits together with resulting clusters

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Segmentation Interface

Muon-arm Hit Reconstruction

irregular segmentation the same technology,

different segmentation layout

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Summary

ALICE uses ROOT based OO framework for simulation and reconstruction (AliRoot)

interface classes provide modularity and coherence of the simulation process

the Monte Carlo interface allows: to build Geant3 and Geant4 application from the same user code to test Geant4 under the same conditions:

geometry, signal generation, output data structures to define Geant3 application in C++

interface classes can be reused in other architectures