1. Operation of the CMS Tracker at the Large Hadron Collider
Thomas Bergauer (HEPHY Vienna)
ÖPG/FAKT Annual Meeting Salzburg
9. September 2010

2. CMS: Compact Muon Solenoid
Supraconducting
Magnet (4 T)
Hadronic
Calorimeter
Electromagnetic
SiTracker
(Pixel and Strips)
Muon System
Magnet
Return Yoke
Very Forward
Weight: t
Diameter: 15 Length: m
9. September 2010
Thomas Bergauer (HEPHY Vienna)

3. CMS Silicon Strip Tracker
Largest silicon tracker built
Active area of 198 m2
5.4 m long, 2.4 m diameter
Components:
Pixel detector (not covered in this talk)
TIB (Inner barrel): 4 layers
TID: 3 Inner Disks
TOB: (Outer Barrel): 6 layers
TEC (Endcaps): 9 disks on each side
Key features:
9.6 Million readout channels
Analog readout
Barrel
(BPIX)
Endcap
(FPIX)
L ~ 90 cm
rmin = 4.4 cm
rmax = 10.2 cm
9. September 2010
Thomas Bergauer (HEPHY Vienna)

4. CMS Tracker in pictures
9. September 2010
Thomas Bergauer (HEPHY Vienna)

5. CMS Tracker Installation December 2007
9. September 2010
Thomas Bergauer (HEPHY Vienna)

6. Thomas Bergauer (HEPHY Vienna)
Angular coverage down to 9 degree to the beam-pipe (|η|<2.5)
4 layers and 3 rings contain stereo modules for 2D hit reconstruction
Basic Building Block:
Detector module
pieces in total
15 different geometries
Modules consist of
Carbon fiber/graphite frame
Front-end hybrid with APV25 readout chips
One or two p-on-n silicon sensor(s), 320/500 micron thick
9. September 2010
Thomas Bergauer (HEPHY Vienna)

7. Thomas Bergauer (HEPHY Vienna)
Readout Chain
9. September 2010
Thomas Bergauer (HEPHY Vienna)

8. Commissioning Procedures
Analog readout
Digitization is done only in off-detector electronics (FEDs)
Thus, detector needs to
Time-align internally (different cable lengths)
Tune laser gain (analog opto-hybrids)
Optimize chip parameters (baseline,…)
Determine noise and pedestals (zero-suppressed data)
Benefit of analog readout
Higher position resolution
Makes debugging easy
9. September 2010
Thomas Bergauer (HEPHY Vienna)

9. Operational fraction of SST
98.1% of channels in operation
TIB/TID: 96.3 %
One ring lost (short, appeared with B field), ~1%
HV missing and HV shorts, ~2.5%
TOB: 98.8 %
One ring lost (short, comes/goes with B field)
TEC: 99.0 %
One HV PG missing (short)
One LV PG missing (short)
TID+
TEC+
TOB
TIB
TID-
TEC-
9. September 2010
Thomas Bergauer (HEPHY Vienna)

10. Signal-to-Noise Ratios
Charge clusters of associated tracks
Divided by noise determined during calibration (pedestal) run
Non-perpendicular tracks normalized by trigonometry
Landau convoluted with Gaussian
MP value taken for summary
TIB
TID
TOB
TEC thin
TEC thick
19.4
18.5
22.5
19.1
23.4
9. September 2010
Thomas Bergauer (HEPHY Vienna)

11. Thomas Bergauer (HEPHY Vienna)
Tracker Alignment
p>3 GeV/c
pT>0.65GeV/c
Only modules with >200 hits
sensors
6 degree of freedom each
O(10mm) accuracy
Minimization hit/track residuals c2
Two approaches:
Millipede (II): Global minimization
“Hits and Impact Points” (HIP): local minimization of sensor position, iterative, detailed track model
Kalman Filter-based fit method working on “correlated” elements, iterative
Applied sequentially
from large substructures to sensor level
Distributions of Mean Residual (DMR): median of the residual distributions in each sensor
2010 cosmics and collision events used for present alignment:
1.5M cosmic tracks (p>4 GeV)
1.7M collision tracks (p>3 GeV)
with constraint to primary vertex
9. September 2010
Thomas Bergauer (HEPHY Vienna)

12. Thomas Bergauer (HEPHY Vienna)
Summary
CMS Tracker (together with whole CMS experiment) performs excellently in both cosmics and pp collision runs
98,1 % channels in operation
Tracker uses analog readout from detector to off-detector electronics
Makes different calibration runs necessary
Signal-to-noise ratio meets expectations
Alignment algorithms reveal accuracy of 10μm
Tracker contributes to the high quality physics data CMS delivers
9. September 2010
Thomas Bergauer (HEPHY Vienna)

13. Thomas Bergauer (HEPHY Vienna)
Thank you for your attention
THE END
9. September 2010
Thomas Bergauer (HEPHY Vienna)

14. APV25 Peak vs. Deconvolution mode
output charge for each strip represents a weighted sum of three consecutive pipeline cells
designed to avoid signal pile-up in high luminosity operations
necessary whenever bunch separation is less than a few hundred nanoseconds
9. September 2010
Thomas Bergauer (HEPHY Vienna)

15. Thomas Bergauer (HEPHY Vienna)
Collected Events
Cosmics muons
2008: 3M tracks in tracker
2009: 4M tracks
2010: 2.2M tracks
~ 4% in pixel detector volume
alignment, calibration, noise, resolution
pp Collisions
Dec 2009 (900GeV+2.36 TeV): ~300k MinBias Events
2010 (7 TeV): ~3000 nb-1
9. September 2010
Thomas Bergauer (HEPHY Vienna)

16. Track Reconstruction Efficency
Tracks reconstructed in three steps:
seeding: hit triplets (mainly pixel hits) or pairs + beam spot used as track candidate
Pattern recognition: track candidate propagation (Kalman filter), addition of new compatible measurements, track candidate cleaning
Final Track Fit: track parameter estimator
Track Selection: fakes rejected with quality cuts
Iterated several times:
hits associated to reconstructed tracks are removed
different seeding algorithms
different quality cuts
9. September 2010
Thomas Bergauer (HEPHY Vienna)

17. Thomas Bergauer (HEPHY Vienna)
Impact parameter
transverse
longitudinal
9. September 2010
Thomas Bergauer (HEPHY Vienna)