Base Classes for Simulation

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

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

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

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

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.


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.




ROOT provides

•

I/O (transient=persistent)

•

C++ Scripting Language

•

Container Classes

•

Automatic Documentation

•

GUI Classes

•

Graphic Tools

•

Histograming Tools

•

….

Base Classes Representing
Simulation Components

Physics Signals

Fast Simulation

Detector
Simulation

Hits


o

Digits


o

Reconstruction

Physics Performance

Feasibility Studies

Detector Performance

Reconstruction Optimisation

Event
-
Generators

Detector Classes

Segmentation

MC

Particle Stack

Example: Fast Simulation

Example: Digitisation


Cluster Finder

Tracking Codes

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;


New Tracking Schema

Detector Version StepManager

Add the hit

GUSTEP

FLUKA Step

Geant4

StepManager

Disk I/O

Objy

Root

AliRun::StepManager

AliMC Implementations



TGeant3


up and running



TGeant4


in development



TFluka


on the wishlist

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.



ALICE G4 Simulation Prototype

Components

Stand
-
Alone

Seen through AliMC

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

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.


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 ...

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 Display


Analysis

Hits and Pad Hits

Hits and PadHits

Hit Reconstruction

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.


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

Event Generator
Implementations


External Generators (Pythia)


External Event Files


Parameterisations (y, pT, particle
cocktail)


Boundary Sources as interface to
FLUKA


Testing Tools: Particle Guns, ...

Interface to FLUKA:

AliGenFLUKASource

FLUKA

AliRoot

ALIFE

Boundary Source

Application: AliGenCocktail:

AliGenCocktail:

public AliGenerator

Tlist



AliGenCocktailEntry

Generator

Events Generated

first/last in stack

biasing

weight

AliGenerator

A fast physics cocktail simulation tool:

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