1. Status of GEANT4 in LHCb
S. Easo,
RAL,
The LHCb experiment.
GEANT4 is used for simulating: RICH testbeam data,
HCAL testbeam data.
GAUSS Project: LHCb Simulation using GEANT4 with GAUDI.
Summary.

2. Precision Measurements of CP violation in the B Meson System.
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.
From the CP asymmetries in the final states of B-meson decays,
Measure CKM Angles.
This design is
being modified
to optimize the
performance of LHCb.

3. To identify charged particles in the momentum range 1-150 GeV/c.
RICH detectors in LHCb
To identify charged particles in the momentum range GeV/c.
Two detectors: RICH1, RICH2.
Momentum range
RICH1: Aerogel 10 GeV/c
C4F < 70 GeV/c
RICH2: CF <150 GeV/c.
Photo Detectors: Baseline solution- HybridPhotodiodes (HPD).
RICH test beam presented: To test the performance of the
Aerogel radiator.

4. Test beam Set-up at CERN
Beam from CERN-PS: πˉ and p/π in the range 6 – 10 GeV/c (Δp/p = 1%)

5. Hybrid Photo Detectors Bialkali photocathode, K2CsSb
Fountain shaped electric field,
demagnification factor ≈ 2.3
Silicon pad sensor 2048 pixels
(16 sectors x 128 pads 1x1 mm²
2.3x2.3 mm² granularity on ph.cathode)
Hybrid
Photo Detectors
AEROGEL test beam
Quantum Efficiency of the 4 photocathodes > 20% (l= nm)

6. Simulation of the Testbeam Setup using GEANT4.
Mirror Rad. of Curvature=1185 mm.
Four Pad Hpds are used.
Hpd
Mirror
Vessel
Filter
Aerogel

7. Optical Transmission in Aerogel
Green Lines: Photons.
Rayleigh Scattered Photons
Photons Transmitted
without Scattering

8. Verification of Aerogel and Filter Transmissions
Generate Photons:
With a uniform wavelength distribution from 170 to 950 nm.
Uniform X and Y coordinates of origin.
With Z coordinate of origin at 180mm (upstream of Aerogel).
With direction along the Z axis.
An Aerogel Tile simulated with: A=0.9368,
C= micrometer**4/cm.
C=Clarity, A=Surface scattering constant.
Transmission = A exp(-C * thickness/ wavelength **4 ).

9. Verification of Aerogel Transmission
Red: Photons incident
on Aerogel Tile
Blue: Photons transmitted
out of aerogel from
the opposite side, but in the
same direction. nm
Black: Blue/Red
Green: Expected
Transmission . nm
Photon wavelength in nm

10. Cherenkov Radiation in Aerogel
Typical Run Configuration in the Testbeam:
9 GeV/c Pions.
One Novosibirsk Aerogel Tile with thickness = 4 cm.
Filter: Glass D263.
Nitrogen Gas at 1 bar and 292 Kelvin in the Vessel.

11. Refractive Index of Aerogel
Novosibirsk Tile 7*8*4 cm.
At 400 nm,
Ref.Index=

12. A Typical event in the Testbeam
Red lines: Charged particle
Green lines : Photons.

13. Cherenkov Radius on the Photocathode
Peak at mm.
Tails from Rayleigh
Scattered Photons.
Radius in mm.

14. Photoelectric Effect at the HPD.
Standard Geant4 processes not applicable in this case.
A Special class created to generate the photoelectrons,
which is derived from a GEANT4 base class.
This process uses the quantum efficiency data
and the results of Fountain focussing tests.
Electron Energy: High Voltage applied.
Direction: From Fountain focussing.
The quantum efficieny data includes the loss of photons by
reflection at the Hpd quartz window surface.

15. Photoelectric Effect at the HPD.
Red lines: Charged particles
Green lines : Photons
HPD Quartz Window,
Silicon detector .

16. Hit Creation in the Si Detector.
Implemented using a special process class since the standard
Geant4 procedure somewhat too complicated for this purpose.
The Photoelectrons loose all their energy in the Silicon.
The backscattering causes a loss of efficiency in creating hits.
Efficiency = B* N/S
where N = threshold cut in terms of width of the pedestal = 4
S= Signal to noise ratio=10
B= backscattering probability=0.18.

17. Test beam results 9 Gev/c π¯ beam 4 cm aerogel Novosibirsk
noise/pad < 2%
Ring region
sect 8
Sector #4
sect 4
Out of ring
Sector #8

18. Photoelectron Yield QE (+- 10%) 10% No Filter Filter D263
Novosibirsk
No Filter
Filter D263
4 cm
9.7 ± 1.0
11.5 ± 1.2
6.3 ± 0.7
7.4 ± 0.8
8 cm
12.2 ± 1.3
14.7 ± 1.6
9.4 ± 1.0
10.1 ± 1.1
Data MC
results are normalised to 2π acceptance
4 cm
(off-ring)
1.13±0.21
0.87 ± 0.09
0.67 ± 0.11
0.55 ± 0.06
8 cm
1.38 ± 0.23
1.34 ± 0.15
1.25 ± 0.21
0.94 ± 0.10
Contributions
to the total error in MC.
QE (+- 10%) 10%
ref. Index variation (+- 5%) 3%
backscattering (+- 2% ) 2%
clarity (+- 2%) 2%
beam divergence (+-1%) 1%
results are in units of 10¯²/cm²
Contributions to total error in real data per HPD
background subtraction (±1σ): 1->8%
inefficient or noisy pads : 2->7%
Extrapolation to full ring : 5%
separation of on-ring/off-ring (±2mm): 5%
signal losses outside ADC thresholds (±1σ): 2%

19. Cherenkov Angle reconstruction
Results per single photoelectron in (mrad):
Thickness
No filter
Filter D263
θc σθ
4 cm
8 cm
Data MC
Aerogel from
Novosibirsk
Data MC
Components of σθ
in mrad for the case with filter.
Pixel size : 1.3
Chromatic: 2.5
Emission Pt: 1.1
Beam divergence: 0.7
Alignment: 2->4 (not included in σθ MC )

20. HCAL Test beam
HCAL is a sampling device made out of steel as absorber and
scintillating tiles are active material.
The scintillating tiles run parallel to the beam axis.
It will provide data for the LHCb hadron trigger.
Using testbeams , the response to particles incident at
various angles is studied and is being compared those from
simulation.

21. Energy Response in HCAL
Response to 50 GeV/c Pions
Histogram : Real Data
Dots: Simulation
Testbeam Data , GEANT3
(MICAP +FLUKA).
HCAL TDR.
Testbeam Data, GEANT4.
I.Belyaev+A.Berdiouguine et. al

22. Energy Resolution of HCAL
Data and G3
Energy Resolution of HCAL
Testbeam Data, GEANT3
GEANT3 with GEISHA, FLUKA,MICAP
Testbeam Data , GEANT4
G4+GEISHA agrees with G3+GEISHA.
Need help to understand and use
G4 with QGS+CHIPS
Data
G4 (QGS+CHIPS)
G4(GEISHA)
Data and G4
I.Belyaev + A. Berdiouguine et. al

23. Status of the GAUSS Project
Current MC productions in LHCb use GEANT3.
GAUSS: To simulate LHCb using GEANT4 .
GIGA interface: to use GEANT4 with the GAUDI Framework.
Ref: Presentation by W. Pokorski on Wednesday.
Geometry Input: XML database. A version available for all
the detectors in LHCb.
Input events: From Pythia or other similar programs
through the HEPMC interface into GEANT4.
A first version of the whole Simulation chain is now working.
Starting to study the response of the detectors in detail.

24. RICH1 with a Particle Gun
RICH1 Event Display
XMLG4OpenGL
Pion with 7 GeV/c.
Cherenkov Photons
In Aerogel and C4F10.
Rayleigh scattering
Switched off for
Illustration.

25. Summary
RICH testbeam simulation is performed using GEANT4.
Results of this simulation is compared with Real Data.
HCAL test beam data comparison with GEANT4 in progress.
A Project to perform the LHCb simulation using
GEANT4 has started.
We are expecting lot of interactions between the GEANT4
collaboration and LHCb in the coming years.