1. 5% 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
TEC=Tracker End-Caps
TOB=Tracker Outer Barrel
TIB=Tracker Inner Barrel
TID=Tracker Inner Disks
Pixel
Z [mm]
R[mm]
Pseudorapidity η
The Compact Muon Solenoid is a general purpose detector designed to study proton proton and lead lead collisions at the LHC.
Silicon Tracker inside the superconducting solenoid for the reconstruction of charged particles, momentum, position and decay verticies.
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
Silicon Microstrip and Pixels
The Tracker Simulation
Description of Geometry
Tracker Inner Barrel
Pixel Barrel
Tracker Inner Barrel
Pixel Forward
Detailed simulation of active and passive volumes
(95% of the total number)
Subdetector
Active Volumes
Passive Volumes
Pixel Barrel (PXB)
768
10201
Pixel Forward (PXF)
672
23670
PIXEL
1440
33871
Inner Tracker (TIB+TID)
3540
56488
Outer Barrel (TOB)
5208
145419
Outer End-Caps (TEC)
6400
113158
Outer Structures
346
STRIP
15148
315411
TRACKER
16588
349283
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)
Material Budget
Comparison with lab measurement
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 5%
Subsystem
Simulation (kg)
Laboratory (kg)
Outer End-Caps (TEC)
691.70
702.22
Inner Tracker (TIB+TID)
427.2
452
Pixel Barrel
2.455
2.598
Ratio Data/Simulation
1.015
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
Validation of Simulation using Cosmics
Charge release in Silicon 288 eV/µm, 3.6 eV/pair e- in t=320 µm
δ-ray cut E> 30 keV (pixel) 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
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
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
Charge Distribution for Pixel Barrel (left) and Endcap (right)
Validation of Simulation using Cosmics
Track Reconstruction Efficiency estimated with three methods (Tracker Barrel)
Tracking resolution estimated after alignment
Based on independent track reconstruction for upper and lower part of cosmic track
Efficiency as a function of transverse momentum for the Combinatorial Track Finder
MC Simulated and measured cluster charge for the Silicon Strip
Tracker corrected for the track incident angle.
RMS of Residuals as a function of transverse momentum for impact parameter (left) and transverse momentum (right)