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The CMS all silicon tracker simulation
Maurizio Biasini - University and INFN Perugia, Italy
On behalf of the CMS Collaboration
The Compact Muon Solenoid
The CMS All Silicon Tracker
TIB=Tracker Inner
Barrel
TID=Tracker Inner Disks
Pixel Barrel
TEC=Tracker End-Caps
Pixel
Silicon Microstrip and Pixels
The CMS Tracker is made of a Silicon
Pixel vertex detector and a Silicon
Microstrip Tracker
• (100 x 150) mm2 pixels
• 320 – 500 mm thick microstrip sensors
• Surface: 200m2 and 1m2
• 10 million Strips and 66 million pixels
TOB=Tracker Outer Barrel
Silicon Tracker inside the
superconducting solenoid for
the reconstruction of charged
particles, momentum, position
and decay verticies.
Tracker Inner Barrel
Pseudorapidity η
R[mm]
The Compact Muon Solenoid is
a general purpose detector
designed to study proton proton
and lead lead collisions at the
LHC.
Z [mm]
The Tracker Simulation
Tracker Inner Barrel
Description of Geometry
Pixel Forward
Detailed simulation of active and passive volumes
(95% of the total number)
Active
Volumes
Pixel Barrel (PXB)
768
Pixel Forward (PXF)
672
PIXEL
1440
Inner Tracker (TIB+TID) 3540
Outer Barrel (TOB)
5208
Outer End-Caps (TEC) 6400
Outer Structures
0
STRIP
15148
TRACKER
16588
Subdetector
The detector simulation is fundamental in optimizing reconstruction
algorithms and in understanding the detector and the first LHC
collinding beam data
Simulation based on Geant4 and CMS OO framework.
Geometry description using Detector Description Language (DDL)
Passive
Volumes
10201
23670
33871
56488
145419
113158
346
315411
349283
Comparison with lab measurement
Material Budget
Each component has been weighted , from the smallest capacitor (mg) to the whole subdetectors (Tons)
Agreement at the 5-10% level found between simulation and measured values
Subsystem
5
%
Simulation (kg) Laboratory (kg) Ratio Data/Simulation
Outer End-Caps (TEC) 691.70
Inner Tracker (TIB+TID) 427.2
Pixel Barrel
2.455
702.22
452
2.598
1.015
1.058
1.058
The average density is 0.17 g/cm3: a MIP loses 35 MeV/m
Barrel region x/X0=0.4: 40% of the photons converts
Simulated Detector Response
Charge release in Silicon
288 eV/µm, 3.6 eV/pair
25000 e- in t=320 µm
δ-ray cut
E> 30 keV (pixel)
120 keV (strip)
Lorentz Angle
Pixel: 23° (120 µm drift)
Strip: 7° (36-61 µm drift)
Charge diffusion
σ≈√Ldrift
(Pixel: 7 µm, Strip: 2 µm)
Electronics Simulation
Electrical chain gain factor
Conversion of the released charge into 6/8-bits ADC counts
Strip: 250 e- = 1 ADC, Pixel: 135 e- = 1 ADC
Electrical Noise
Gaussian noise is added
(Pixel: σ̄=350 e-, Strip: σ̄=1200 e-)
Noise increase with radiation damage (even at operation
temperature T=-20°C): conservative +50%
AC couplings
Inter-strip coupling: 3%-1% (11%-7%) of the charge fraction
assigned to the neighbours strips for TOB-TIB in peak
(deconvolution) mode
Validation of Simulation using Cosmics
The electronics pile-up is
simulated processing the
signals of the preceding 5 and
fhe ollowing 3 bunch crossings
when simulating the actual
bunch crossing (25 ns)
Strips: two readout modes,
signal shape in deconvolution
reduces the pile-up
First full Tracker commissioning Cosmic Run At Four Tesla CRAFT 08
270 M cosmic events selected – 6.5 M with track in Tracker – 3.2M/110K
high quality tracks for Strip/Pixels
Possibility to check and validate Tracker Simulation
Charge Distribution for Pixel Barrel (left) and Endcap (right)
Validation of Simulation using Cosmics
Track Reconstruction Efficiency estimated
with three methods (Tracker Barrel)
MC Simulated and measured cluster charge for the Silicon Strip
Tracker corrected for the track incident angle.
Efficiency as a function of transverse
momentum for the Combinatorial
Track Finder
Tracking resolution estimated after alignment
Based on independent track reconstruction for upper and lower part of cosmic track
RMS of Residuals as a function of transverse momentum for impact
parameter (left) and transverse momentum (right)