1. GAUSS - GEANT4 based simulation for LHCb
GEANT4 Users’ Workshop
14 November 2002
W. Pokorski / CERN

2. Contents Introduction to LHCb Overview of Gauss project
GiGa – Gaudi interface to GEANT4
overview and presentation of a few selected topics
Summary
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LHCb Simulation

3. LHCb Experiment
Precision Measurements of CP violation in the B Meson System
From the CP asymmetries in the final states of B-meson decays,
measure CKM Angles.
Large Sample of Events with Bd and Bs Mesons
Most of the b hadrons are produced at small polar angles.
LHCb: Single Forward Arm Spectrometer with Open Geometry.
This design is
being modified
to optimize the
performance of LHCb.
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4. GAUDI – LHCb software framework
all of the LHCb event processing software is built within a framework – Gaudi framework
separation between data and algorithms
data store-centred architectural style
separation between transient and persistent data
isolation of user codes/algorithms from underlying persistency technologies
components interact through their abstract interfaces
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5. GAUDI - Object Diagram Converter Interactive Service Application
Manager
Converter
Converter
Event
Selector
Transient Event Store
Data
Files
Message
Service
Persistency
Service
Event Data
Service
JobOptions
Service
Algorithm
Algorithm
Algorithm
Data
Files
Transient Detector
Store
Particle Prop.
Service
Persistency
Service
Detec. Data
Service
Other
Services
Data
Files
Transient
Histogram Store
Persistency
Service
Histogram
Service
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6. Gauss application Int.face GiGa Digi Alg Geant4 (GiGa) Pythia etc
JobOpts
JobOpts
JobOpts
Int.face
GiGa
Digi
Alg
Geant4
(GiGa)
Pythia
etc
Geant4
MCParticle
MCVertex
MCHit
Digit
MCDigit
HepMC
Cnv
Cnv
Cnv
Geometry
Generator
Detector Simulation
Digitization
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7. GiGa overview
GEANT4 Interface for Gaudi Applications or Gaudi Interface to GEANT4 Applications
makes GEANT4 callable and controllable from within GAUDI environment
common detector geometry source used by other applications (reconstruction, visualisation)
communication via Transient Stores (Event, Detector Data) as any other service or algorithm in Gaudi
use of common services (ParticlePropertySvc, RandomNumberSvc, MagneticFieldSvc, etc.)
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8. GiGa structure Data Files Persistency Service Application Manager
GiGaKine
Conversion
Service
G4 Kine
Transient Event Store
Event
Service
Geant4
GiGaHits
Conversion
Service
G4 Hits
Algorithm
Converter
Algorithm
Cnv
GiGa
Service
Algorithm
Cnv
Transient Detector
Store
GiGaGeom
Conversion
Service
G4 Geom
Action
Detec.
Service
Action
we can see two parts of GiGa
GiGaCnv – converts data (geometry, events) from and to transient stores (and loads it/extracts it to/from G4)
GiGa – configures, triggers and control the actual simulation
Other
Services
Data
Files
Persistency
Service
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9. GiGa features it is a façade pattern minimizes the couplings to Geant4
all interactions with Geant4 only through abstract interfaces of GiGa Service
provides access to internal G4 event loop via GiGaRunManager
instantiates different “actions” and physics lists using abstract factory approach (makes them to be plugable components)
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10. A few selected topics:
Dynamic loading and instantiation using Abstract Factories
GiGaRunManager
GaussTrajectory and GaussEventAction
Loading the Geometry
Sensitive Detectors
Physics lists
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11. Abstract Factory approach (1/3)
concrete implementations of different functional parts (user actions, physics lists, etc) are collected in “component libraries”
not linked to the main program, used purely at run-time
changes in the implementation of the component library does not require the application to be relinked
configurable at run-time
set of components (different UserActions, Physics lists, etc) can be changed by specifying different jobOptions of the application
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12. Abstract Factory approach (2/3)
List of dlls to load
Table of components that could be instantiated
DLL
DLL
getFactoryTable
DLL
ApplicationMgr
FactoryTable
Set of options for
components
(for ex cuts for
physics list)
Factories which are in charge of instantiating concrete objects
{instantiate}
SvcFactory
SvcFactory
xxxFactory
IFactory
“new” executed here!!!
{new}
Application
Service
Service
UserActions,
Physics Lists,
etc
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13. Abstract Factory approach (3/3)
Gaudi world
G4 world
virtual
Gaudi Services
Abstract Factory returns pointer to this
concrete implementations of components
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14. GiGaRunManager GiGaRunManager is a specialization of G4RunManager
no beamOn method, “event loop” external to GiGaRunManager, no “G4 run” concept
simulation of single events triggered (and controlled) externally
calls directly to G4EventManager::processOneEvent
construction of primary events controlled from outside of G4
“event building” controlled from jobOptions
no UserPrimaryGeneratorAction::GeneratePrimaries
direct control over calls to G4Event::AddPrimaryVertex
possibility of combining several generated events (pile-up) inside one G4 events by setting appropriate jobOptions
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15. GaussTrajectory implements G4VTrajectory
in addition to G4Trajectory: time of flight at each TrajectoryPoint – needed in order to convert to LHCb event model
speciallized AppendStep method – triggered by a flag set in GaussStepAction::UserSteppingAction
controls which trajectory points to append (begin, end, reflections of optical photons, creation of interesting particles, etc)
storing (or not) of trajectories controlled by GaussTrackingAction
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16. GaussTracking Action
PostUserTrackingAction: takes decision whether to store given trajectory
several criteria implemented (to be extended) depending on:
energy, energy of secondaries
particle type, particle type of secondaries
creation of a hit
etc
in case a trajectory is not stored, consistency is assured by updating ParentID of the secondaries to the last (in given decay chain) stored trajectory
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17. GiGa - Geometry Conversion
Xml description
Materials
Volumes
Xml Cnv
Geo Conversion Service
Converter
Converter
Gaudi transient store
Geant4
Materials
Geant4
Volumes
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18. Geometry conversion ex. (RICH 1)
XmlGaudiGiGaG4OpenGL
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19. Sensitive Detectors & Hits
GiGaSensDetTracker
G4TrackerHits
(Geant4)
creates
GiGaTrackerHitsCnv
ProcessHit()
Geant4 world
converts to:
invoked when particle passed through the sensitive volume
MCHits
(/Event/MC/OT/Hits)
lvVolume
(XmlDDDB)
Gaudi world
<logvol name="lvU_ActiveLayer" … sensdet="GiGaSensDetTracker/myDet">
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20. Detector Simulation – “physics lists”
crucial part of the whole simulation program – will certainly require several tuning iterations
most of the stuff already implemented in Geant4
some specific processes needed implementation
for RICH: photoelectric process (creation of photoelectrons in HPDs), energy loss: in the silicon of HPDs
modular physics lists implemented using Abstract Factories concept
allows dynamic loading and configuration (via jobOptions) of different physics lists, not need to recompile anything
expected to increase flexibility and to make validation easier
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LHCb Simulation

21. RICH1 with SingleParticleGun
RICH1 Event
Pion with 7 GeV/c.
Cherenkov Photons
In Aerogel and C4F10.
Rayleigh scattering
Switched off for
Illustration.
S. Easo
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22. RICH1 Hits
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23. Panoramix view of MCHits
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24. Panoramix view of MCHits (2)
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25. Summary
LHCb simulation application build within common LHCb software framework (used by reconstruction, visualization, analysis)
communication with GEANT4 via an interface (GiGa) allowing flexible use of G4 functionalities
extensive use of AbstractFactory concept, allowing run-time loading of specific components
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LHCb Simulation