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Simulation Application for the LHCb Experiment
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Contents

• Introduction to LHCb
• Overview of Gauss project
• GiGa Gaudi interface to GEANT4
• Physics validation
• Summary

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LHCb Experiment

• Precision Measurements of CP violation in the B
  Meson System

• 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
• Measure CKM angles from the CP asymmetries in
  the final states
• of B-meson decays

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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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GAUDI - Object Diagram
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Gauss application overall structure

• Event generation
• primary event generator (Pythia, Herwig, etc)
• specialised decay package (EvtGen,...)
• pile-up generation
• Detector simulation
• geometry (LHCb -gt G4)
• tracking through materials (G4)
• hit creation (G4)
• MC truth generation
• Digitization
• subdetector specialised algorithms
• detector response simulation

Generation Sequence
HepMC
Simulation Sequence
time
MCevent
Digitization Sequence
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Gauss application - design
GiGa
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GiGa overview

• 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)
• 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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GiGa structure
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GiGa features

• its a Gaudi service
• provides access to internal Geant4 event loop
  via GiGaRunManager
• all interactions with Geant4 only through
  abstract interfaces of GiGa Service
• minimizes the couplings to Geant4
• allows loading external physics lists
• instantiates (using Abstract Factory pattern)
  different actions (makes them to be plugable
  components)

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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
ltlogvol name"lvU_ActiveLayer"
sensdet"GiGaSensDetTracker/myDet"gt
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Detector Simulation physics lists

• 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
  validation easier

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Physics validation

• comparison to the test-beam data
• validation of Rayleigh scattering, Cherenkov
  radiation, photoelectric effect, energy loss in
  the Silicon for RICH detectors
• studies of different hadronic physics models in
  hadronic calorimeter simulations
• comparison to the Geant 3 simulation
• occupancies, momentum distributions, time of
  flight, energy deposition distributions, etc

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Vertex Locator physics validation (1/2)
Geant3 Geant4
Geant3 Geant4
Energy deposition in the silicon (eV)
Number of hits per event
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Vertex Locator physics validation (2/2)
Geant3 Geant4
Geant3 Geant4
Time of Flight (ns)
Particle energy (GeV)
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Outer Tracker physics validation (1/2)
Geant3 Geant4
Number of hits per event
XY distribution
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Outer Tracker physics validation (2/2)
Geant3 Geant4
Momentum distribution (GeV)
Time Of Flight (ns)
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Summary

• Current Status
• we can run GEANT4 simulation with all the
  subdetectors included (complete LHCb detector
  geometry), realistic Pythia events and Geant4
  proposed physics list
• we are testing/validating subdetector specific
  code and physics processes
• Planning
• large test productions starting from the summer
• definite move from the old Geant3 simulation to
  the new Geant4 based one, early next year

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