Status of GEANT 4 in LHCb S Easo

Status of GEANT 4 in LHCb S. Easo, RAL, 30 -9 -2002 • The LHCb experiment. • GEANT 4 is used for simulating: RICH testbeam data, HCAL testbeam data. • GAUSS Project: LHCb Simulation using GEANT 4 with GAUDI. • Summary.

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.

RICH detectors in LHCb • To identify charged particles in the momentum range 1 -150 Ge. V/c. • Two detectors: RICH 1, RICH 2. Momentum range RICH 1: Aerogel C 4 F 10 2 10 Ge. V/c < 70 Ge. V/c RICH 2: CF 4 <150 Ge. V/c. • Photo Detectors: Baseline solution- Hybrid. Photodiodes (HPD). • RICH test beam presented: To test the performance of the Aerogel radiator.

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

Hybrid Photo Detectors • Bialkali photocathode, K 2 Cs. Sb • Fountain shaped electric field, demagnification factor ≈ 2. 3 • Silicon pad sensor 2048 pixels (16 sectors x 128 pads 1 x 1 mm² 2. 3 x 2. 3 mm² granularity on ph. cathode) AEROGEL test beam Quantum Efficiency of the 4 photocathodes > 20% (l=280 -380 nm)

Simulation of the Testbeam Setup using GEANT 4. Mirror Rad. of Curvature=1185 mm. Four Pad Hpds are used. Hpd Mirror Vessel Filter Aerogel

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

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 180 mm (upstream of Aerogel). • With direction along the Z axis. An Aerogel Tile simulated with: A=0. 9368, C=0. 00719 micrometer**4/cm. C=Clarity, A=Surface scattering constant. Transmission = A exp(-C * thickness/ wavelength **4 ).

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

Cherenkov Radiation in Aerogel • Typical Run Configuration in the Testbeam: • 9 Ge. V/c Pions. • One Novosibirsk Aerogel Tile with thickness = 4 cm. • Filter: Glass D 263. • Nitrogen Gas at 1 bar and 292 Kelvin in the Vessel.

Refractive Index of Aerogel Novosibirsk Tile 7*8*4 cm. At 400 nm, Ref. Index=1. 03066.

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

Cherenkov Radius on the Photocathode Peak at 146. 4 mm. Tails from Rayleigh Scattered Photons. Radius in mm.

Photoelectric Effect at the HPD. • Standard Geant 4 processes not applicable in this case. • A Special class created to generate the photoelectrons, which is derived from a GEANT 4 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.

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

Hit Creation in the Si Detector. • Implemented using a special process class since the standard Geant 4 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 = 1. 0 - 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.

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

Photoelectron Yield Novosibirsk No Filter D 263 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) 8 cm (off-ring) 1. 13± 0. 21 0. 67 ± 0. 11 0. 87 ± 0. 09 0. 55 ± 0. 06 1. 38 ± 0. 23 1. 25 ± 0. 21 1. 34 ± 0. 15 0. 94 ± 0. 10 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 (± 2 mm): 5% signal losses outside ADC thresholds (± 1σ): 2% 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%

Cherenkov Angle reconstruction • Results per single photoelectron in (mrad): Thickness No filter Filter D 263 θc θc σθ σθ 4 cm 250. 0 248. 7 5. 4 4. 0 247. 1 246. 8 5. 0 3. 1 8 cm 246. 8 245. 0 5. 8 3. 9 245. 4 243. 7 4. 8 3. 0 Components of Pixel size : 1. 3 Chromatic: 2. 5 Emission Pt: 1. 1 Data MC Aerogel from Novosibirsk σθ in mrad for the case with filter. Beam divergence: 0. 7 Alignment: 2 ->4 (not included in σθ MC )

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.

Energy Response in HCAL Response to 50 Ge. V/c Pions Histogram : Real Data Dots: Simulation • Testbeam Data , GEANT 3 (MICAP +FLUKA). • HCAL TDR. • Testbeam Data, GEANT 4. I. Belyaev+A. Berdiouguine et. al

Data and G 3 Energy Resolution of HCAL • Testbeam Data, GEANT 3 • GEANT 3 with GEISHA, FLUKA, MICAP • Testbeam Data , GEANT 4 • G 4+GEISHA agrees with G 3+GEISHA. • Need help to understand use G 4 with QGS+CHIPS Data and G 4 • Data § G 4 (QGS+CHIPS) G 4(GEISHA) I. Belyaev + A. Berdiouguine et. al

Status of the GAUSS Project • Current MC productions in LHCb use GEANT 3. • GAUSS: To simulate LHCb using GEANT 4. • GIGA interface: to use GEANT 4 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 GEANT 4. • A first version of the whole Simulation chain is now working. • Starting to study the response of the detectors in detail.

RICH 1 with a Particle Gun RICH 1 Event Display XML G 4 Open. GL Pion with 7 Ge. V/c. Cherenkov Photons In Aerogel and C 4 F 10. Rayleigh scattering Switched off for Illustration.

Summary • RICH testbeam simulation is performed using GEANT 4. • Results of this simulation is compared with Real Data. • HCAL test beam data comparison with GEANT 4 in progress. • A Project to perform the LHCb simulation using GEANT 4 has started. • We are expecting lot of interactions between the GEANT 4 collaboration and LHCb in the coming years.