1. LHCb-Italian Software Tutorial
Updated on 13 July 2007
Detector Simulation
Gloria Corti
CERN
Gauss Tutorial

2. Outline Overview of (Physics) Detector Simulation Purpose: reminder
Interaction with (use of) Geant4
Job options
Detectors
Physics processes
Input to and Output from Geant4
Output Data
Software Tutorial - Gauss - Last update: 10 October 2007

3. Purpose: a reminder For Detector Simulation phase of Gauss
tracking of particles in the detector and interactions with the material
production of “hits" when particles cross sensitive detectors
Data produced can be studied directly or in further processing
MCTruth as seen by the (LHCb) experimental setup
MCParticles and MCVertices
MCHits (Velo, PuVeto, TT, IT, OT, Muon system), MCRichHits and MCCaloHits
Output (.sim ) can be processed by Boole for digitization
Software Tutorial - Gauss - Last update: 10 October 2007

4. Simulation phase reading generator event
Options are available in Gauss to run the simulation phase reading in the generator event from a file previously produced (although not really necessary from the CPU point of view)
../slc3_ia32_gcc323/Gauss.exe $GAUSSOPTS/v SimPhase.opts //
// Phases to be executed:
ApplicationMgr.TopAlg += { “GaudiSequencer/Simulation” };
// Generator Phase
#include "$GAUSSOPTS/Generator.opts“
Generator.MeasureTime = true;
#include "$GAUSSOPTS/GenStandAlone.opts“
// Simulation Phase
// #include "$GAUSSOPTS/Simulation.opts"
// Simulation.MeasureTime = true;
Software Tutorial - Gauss - Last update: 10 October 2007

5. Simulation phase reading generator event
Pythia,
EvtGen
JobOpts
Interface …
Initialize
JobOpts
HepMC
MCParticle
MCVertex
MCHits
POOL
LHCb Event model
Exchange model
GiGa
Geant4
Init
HepMC
Monitor
Cnv
Cnv
Cnv
POOL
Geometry
Event Generation
primary event generator
specialized decay package
pile-up generation
Detector Simulation
geometry of the detector (LHCb  Geant4)
tracking through materials (Geant4)
hit creation and MC truth information (Geant4  LHCb)
Software Tutorial - Gauss - Last update: 10 October 2007

6. Geant4
Gauss uses GEANT4 for transporting particles in the experimental setup and simulating the physics processes that can occur.
Navigation in EM fields
Physics processes for a variety of particles at different energies
GEANT4 is the successor of GEANT3, the world-standard toolkit for HEP detector simulation
GEANT4 is a C++ toolkit developed in the Physics community
International Collaboration ~ 10 year old
used in HEP, nuclear physics, heavy ion physics, cosmic ray physics, astrophysics, space science and medical applications
GEANT4 coverage of physics comes from mixture of theory-driven, parameterized, and empirical formulae. Both cross-sections and models (final state generation) can be combined in arbitrary manners as necessary.
Standard and Low energy EM processes, Hadronic processes, Optical photon processes, Decay processes, etc.
Documentation from web: Gauss → Geant4 → Geant4 6
Software Tutorial - Gauss - Last update: 10 October 2007

7. GiGa
Gauss uses a dedicated Gaudi service to interact and communicate with Geant4
minimizes the couplings to Geant4
GIGA = GEANT4 Interface for Gaudi Applications or Gaudi Interface to GEANT4 Applications
GEANT4 callable and controllable from within GAUDI environment
common detector geometry source used by other applications (reconstruction, visualisation)
use of Gaudi features as algorithms, tools, services
use of common services (ex. RandomNumberSvc, MagneticFieldSvc, DetectorDataSvc, etc.)
access to internal Geant4 event loop via GiGaRunManager
allows loading external physics lists
instantiates (using Abstract Factory pattern) different “actions” (makes them to be plugable components)
Documentation on the web from Gauss web page 6
Software Tutorial - Gauss - Last update: 10 October 2007

8. Encapsulation of Geant4 in Gauss via GiGa
Single point of connection
to Geant4
Convertion of transient objects to/from Geant4 representation
Detector geometry converted from same source as other applications
Data
Files
Persistency
Service
G4 Kine
G4 Hits
Transient Event Store
Kine
Conversion
(Algorithms)
Geant4
Hits
Conversion
(Algorithms)
GiGa
Service
Gaudi
GiGa
Transient Detector
Store
GiGaGeom
Conversion
Service
G4 Geom
Event Loop controlled by Gaudi
Action
Action
Embedded Geant4, Geant4 control and Configuration
Persistency
Service
Data
Files
Software Tutorial - Gauss - Last update: 10 October 2007

9. Control of simulation phase via JopOptions (1)
Simulation.opts is the steering options file
configuration of algorithms in Simulation phase
// Pass HepMCEvents to Geant4
Simulation.Members += { "GeneratorToG4" };
// Perform the Geant4 simulation
Simulation.Members += { "GiGaFlushAlgorithm" };
// Check the G4Event processing status
Simulation.Members += { "GiGaCheckEventStatus" };
// Populate the MCParticles/MCVertices in the TES
Simulation.Members += { "G4HepMCToMCTruth" };
// Populate sensitive hits in the TES
Simulation.Members += { "GaudiSequencer/DetectorsHits" }; DetectorsHits.Members += { "GetTrackerHitsAlg/GetVeloHits" };
DetectorsHits.Members += { "GetTrackerHitsAlg/GetTTHits" };
...
DetectorsHits.Members += { "GetCaloHitsAlg/GetSpdHits" };
DetectorsHits.Members += { "GetCaloHitsAlg/GetEcalHits" };
DetectorsHits.Members += { "GetMCRichHitsAlg" };
#include “$GAUSSTRACKERROOT/options/TrackersHits.opts”
Software Tutorial - Gauss - Last update: 10 October 2007

10. Control of simulation phase via JopOptions (2)
configuration of GiGa to control Geant4 actions
// Mandatory run action
GiGa.RunAction = "GiGaRunActionSequence/RunSeq";
GiGa.RunSeq.Members += { "TrCutsRunAction/TrCuts"};
GiGa.RunSeq.Members += {
"GiGaRunActionCommand/RunCommand" };
GiGa.RunSeq.RunCommand.BeginOfRunCommands =
{ "/tracking/verbose 0",
"/tracking/storeTrajectory 1",
"/process/eLoss/verbose -1", };
//GiGa.RunSeq.RunCommand.BeginOfRunCommands += { "/process/list" };
// Mandatory event action
GiGa.EventAction = "GiGaEventActionSequence/EventSeq";
GiGa.EventSeq.Members += { "GaussEventActionHepMC/HepMCEvent" };
setting cuts
MC truth internal in G4
Software Tutorial - Gauss - Last update: 10 October 2007

11. GetTrackerHits configuration for TT
Detector simulation
GiGaSensDetTracker
creates
G4TrackerHits
GetTrackerHitsAlg
ProcessHit()
Geant4 world
converts to:
invoked when particle passed through the sensitive volume
lvVolume
as in XmlDDDB
MCHits in TES
/Event/MC/OT/Hits
Gaudi world
<logvol material “Silicon”
name="lvLadder1"
sensdet="GiGaSensDetTracker/TTSDet">
GetTrackerHits configuration for TT
GetTTHits.CollectionName = “TTDSet/Hits”;
GetTTHits.MCHitsLocation = “/Event/MC/TT/Hits”;
GetTTHits.MCHitsLocation = “/dd/Structure/LHCb/BeforeMagnetRegion/TT”; 9
Software Tutorial - Gauss - Last update: 10 October 2007

12. Control of Detector simulation
In SimGeometry.opts
included from Simulation.opts
uses a special GiGa component
list of detectors to simulate
Can give different list: switch off some detectors, add some new, Test Beam setup
Just need the sensdet Keyword in the XML Detector Description
Simulation.Members += { "GiGaInputStream/Geo" };
#include "$GAUSSOPTS/SimGeometry.opts"
Geo.ConversionSvcName = "GiGaGeo" ;
Geo.DataProviderSvcName = "DetectorDataSvc" ;
Geo.StreamItems += {"/dd/Structure/LHCb/BeforeMagnetRegion/Velo"};
Geo.StreamItems += {"/dd/Structure/LHCb/BeforeMagnetRegion/Velo2Rich1"};
Geo.StreamItems += {"/dd/Structure/LHCb/BeforeMagnetRegion/Rich1"};
Geo.StreamItems += {"/dd/Geometry/BeforeMagnetRegion/Rich1/Rich1Surfaces"};
...
// Geo.StreamItems += {"/dd/Structure/LHCb/DownstreamRegion/Hcal"};
// Geo.StreamItems += {"/dd/Structure/LHCb/DownstreamRegion/Muon"};
Software Tutorial - Gauss - Last update: 10 October 2007

13. Physics processes and lists
Geant4 has a big variety of processes that can be combined as necessary in Physics Lists:
crucial part of the whole simulation program
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
GiGa modular physics lists
allows dynamic loading (via jobOptions) of particular physics “sublists”
expected to increase flexibility and to make changes easier 10
Software Tutorial - Gauss - Last update: 10 October 2007

14. Configuration of Physics lists to use
Defined as list of options that can be changed
In MuonLowEnergy.opts for example:
GiGa.ModularPL.PhysicsConstructors +=
{ "GiGaExtPhysics<GeneralPhysics>/GeneralPhysics" } ;
{ "GiGaExtPhysics<EMPhysics>/EMPhysics" } ;
{ "GiGaExtPhysics<MuonPhysics>/MuonPhysics" } ;
{ "GiGaExtPhysics<HadronPhysicsLHEP>/LHEPPhysics" } ;
{ "GiGaExtPhysics<IonPhysics>/IonPhysics" } ;
{ "GiGaPhysConstructorOp" } ;
{ "GiGaPhysConstructorHpd" } ;
LHCb RICH processes
set a different hadronic physics list with neutrons to thermal energy
GiGa.ModularPL.PhysicsConstructors -=
{ "GiGaExtPhysics<HadronPhysicsLHEP>/LHEPPhysics" } ;
GiGa.ModularPL.PhysicsConstructors +=
{ "GiGaExtPhysics<HadronPhysicsQGSP_BERT_HP>/QGSP_BERT_HPPhysics" };
Software Tutorial - Gauss - Last update: 10 October 2007

15. Geant4 production thresholds
Geant4 has production thresholds for EM processes
Specify range (which is converted to energy for each material) at which continuous loss begins, track primary down to zero range
Create secondaries only above specified range, or add to continuous loss of primary for secondaries of less energetic than travelling the required range in the current material
Energy cut = 450 KeV
Range cut 1.5 mm
(450 KeV in liq.Ar, 2 MeV in Pb)
liq.Ar Pb
liq.Ar Pb
Software Tutorial - Gauss - Last update: 10 October 2007

16. Energy threshold below which the particle is not tracked.
LHCb tracking cuts
Introduce tracking cuts on Ekin of particles of all type (special Gauss stepping actions)
Track particle until cut-off energy is reached, stop it at that point
Possible also to set cuts per region
For new geometry for example to turn on delta rays in RICH1 Aerogel (set in Simulation.xml)
GiGa.RunSeq.Members += { "TrCutsRunAction/TrCuts"};
Effect of cut value for ECAL with 30 GeV electron particle gun
Processing time
Energy threshold below which the particle is not tracked.
Software Tutorial - Gauss - Last update: 10 October 2007

17. Control of cuts (1) In job options for overall cuts
// Default production cuts for whole detector
GiGa.ModularPL.CutForElectron = * mm;
GiGa.ModularPL.CutForPositron = 5.0 * mm;
GiGa.ModularPL.CutForGamma = 10.0 * mm;
// Default tracking cuts for whole detector
GiGa.RunSeq.TrCuts.MuonTrCut = 10.0*MeV;
GiGa.RunSeq.TrCuts.pKpiCut = 10.0*MeV;
GiGa.RunSeq.TrCuts.NeutrinoTrCut = 0.0*MeV;
GiGa.RunSeq.TrCuts.NeutronTrCut = 10.0*MeV;
GiGa.RunSeq.TrCuts.GammaTrCut = 1.0*MeV;
GiGa.RunSeq.TrCuts.ElectronTrCut = 1.0*MeV;
GiGa.RunSeq.TrCuts.OtherTrCut = 0.0*MeV;
Production cuts
Tracking cuts
Software Tutorial - Gauss - Last update: 10 October 2007

18. Control of cuts (2)
In Simulation-v xml for detector specific cuts
<SimAtt name="SimAttrMuFilt"
minEkine=“500.0*MeV"/>
<SimAtt name="SimAttrMuon"
minEkine="10.0*MeV"/>
<Item name="/dd/Geometry/DownstreamRegion/Muon/lvMuFilter1">
<Cut particle="0" attr="SimAttrMuFilt"/>
<Cut particle="13" attr="SimAttrMuon"/>
<Cut particle="-13" attr="SimAttrMuon"/>
</Item>
Software Tutorial - Gauss - Last update: 10 October 2007

19. Understanding the simulation as a whole
To understand in depth the simulation, the information about the history of what happened in the detector is very important (trajectories and processes originating them)
MC truth filled at the end of an event processing by Geant4
Necessary to decide a priori what to store
when a particle is created, when it is being processed, when it stops being “tracked”
Set of criteria can be changed in job options
GiGa.TrackSeq.PostTrack.StoreAll = false;
GiGa.TrackSeq.PostTrack.StorePrimaries = true;
GiGa.TrackSeq.PostTrack.StoreForcedDecays = true;
GiGa.TrackSeq.PostTrack.StoreMarkedTracks = true;
// The following only for z=12280mm (i.e.PRS/SPD)
GiGa.TrackSeq.PostTrack.StoreByOwnEnergy = true;
GiGa.TrackSeq.PostTrack.OwnEnergyThreshold = * MeV;
GiGa.TrackSeq.PostTrack.StoreByChildProcess = true;
GiGa.TrackSeq.PostTrack.StoredChildProcesses = {"RichG4Cerenkov"};
GiGa.TrackSeq.PostTrack.StoreByOwnProcess = true;
GiGa.TrackSeq.PostTrack.StoredOwnProcesses = {“Decay”};
pp collision
by detectors
with E > threshold
producing Cerenkov
from decay in detector
Software Tutorial - Gauss - Last update: 10 October 2007

20. History information after simulation
History of particles traveling trough detector in dedicated LHCb event data classes: MCParticles and MCVertices
Derive from basic LHCb event data class: KeyedObject
Container support
Transient and Persistent support
Link from other classes ( MCHits for example, but also possibility to associate them to reconstructed Tracks, physics Particles)
Relationship between them holds the tree structure
An MCParticle has a SmartRef<MCVertex> originVertex() and SmartRefVector<MCVertex> endVertices()
more than one end vertex (Bremsstrahlung)
An MCVertex has a SmartRef<MCParticle> mother and SmartRefVector<MCParticles> products
a primary vertex does not have a mother MCParticle
Keep originating process identifier in MCVertex
These classes are written out by the simulation and accessed in a variety of ways in successive processing
Software Tutorial - Gauss - Last update: 10 October 2007

21. Output Files and extra information
List of output data objects in normal production can be found in GaussTape.opts
Sometimes necessary to have additional information for special studies: ex. RICH extended info
To get the extra information and write it to tape uncomment $GAUSSOPTS/RichExtendedInfo.opts in Simulation.opts
DetectorsHits.Members +={ "GetMCRichOpticalPhotonsAlg“,
“GetMCRichSegmentsAlg”, “GetMCRichTracksAlg” };
#include "$GAUSSRICHROOT/options/RichExtendedInfo.opts"
GaussTape.OptItemList += {
“/Event/MC/Rich/OpticalPhotons#1”
,”/Event/MC/Rich/Tracks#1”
,”/Event/MC/Rich/Segments#1” };
Software Tutorial - Gauss - Last update: 10 October 2007

22. Exercises You will get familiar with some controls of the simulation
Don’t forget you can always read a file you produced
Guidelines for the exercises on the web
from the tutorial agenda
Packages used:
Sim/Gauss v25r12 – Mandatory
Tutorial/Simulation v2r0 – Optional
exercises/README.txt + exerciseN.txt
solutions/exerciseN/
Advanced exercises also available
Software Tutorial - Gauss - Last update: 10 October 2007