1. Integration and alignment of ATLAS SCT
Roland Härtel
IMPRS Seminar
May 12, 2006

2. Content Introduction (LHC, ATLAS, InnerDetector)
SCT (layout, principle of operation)
Beam tests ( )
Alignment
Combined Test Beam (2004)
IMPRS Seminar Roland Härtel Integration and alignment of ATLAS SCT

3. Large Hadron Collider 27 km long accelerator tunnel
bending radius of 2804 m
pp center-of-mass energy: 14 TeV
25 ns between collisions (40 MHz)
high radiation environment at interaction points
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

4. ATLAS InnerDetector
Inner Detector used for particle track reconstruction
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

5. SCT Barrel and Endcap SCT = SemiConductorTracker
SCT is the silicon strip detector of ATLAS and consist of 4088 modules
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

6. SCT Barrel and Endcap modules
each SCT module has 2 x 768 readout strips
the EndCap modules are wedge shaped due to the radial arrangement on a disk
stereo angle of 40 mrad between readout sides
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

7. SCT operation principle
pn diode, operated in reverse biased mode (150 V)
charge collection in p+-strips
readout via mirror charge in aluminum readout strips
after amplification, shaping and discrimination only a binary signal is retained
discrimination threshold  1 fC
strip pitch is 80 m
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

8. SCT coordinate system
Coordinate system of SCT module is a right-handed orthogonal three-dimensional frame that is aligned with one readout side (r--side)
The origin of the coordinate system is at the center-of-gravity of the r--side
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

9. Beam tests of SCT modules - Setup
Beam tests with CERN SPS + beam (180 GeV) with and without 1.56 T magnetic field
Beam telescope detectors were used for tracking and spatial resolution measurements (uncertainty  5 m)
Sets of 12 SCT modules were tested
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

10. Beam test - residuals Barrel Endcap
Binary readout creates top hat shaped residual distributions
rms = 23 m = pitch / 12
space point resolution: x = 16 m y = 850 m
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

11. Beam test – discrimination threshold
hit efficiencies > 99% and noise occupancy < 5*10-4 are required
for non-irradiated modules this can be easily achieved
irradiated SCT modules have only a very narrow discrimination threshold region where these requirements are met
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

12. Beam test – charge collection
Theta Phi
The incident angle of a track has a great influence on charge sharing between neighboring readout strips
The 10° tilt angle of SCT barrel modules results in a 20 % multi-strip cluster probability
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

13. Beam test – Lorentz angle
In magnetic field the holes that drift to p+-strips are subject to Lorentz force.
This offset can be expressed in terms of an angle
magnetic field of beam test: 1.56 T
L (150 V) = 3.3°
L (350 V) = 2.1°
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

14. Alignment Process of deducing position of each SCT module
Misalignments can be corrected for in track reconstruction software
Modules are not moved back into nominal position
Alignment accuracy must match intrinsic spatial resolution to minimize systematic errors
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

15. Track based alignment
The residual, i.e. the distance between fitted particle trajectory and the readout strip is used to measure misalignments
distorted and not centered residual distributions can be the effect of misalignments
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

16. Iterative approach
For each module residuals and derivatives are determined, the complete geometry information is encoded there
With a linear least squares method the most probable values for the alignment parameters are determined
Updated alignment parameters are then used in the next iteration cycle to calculate new residuals and derivatives.
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

17. Alignment parameters – SCT barrel
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

18. Alignment accuracy SCT
Achievable alignment accuracy after 10 iterations with 150k tracks from a single pion sample.
Only SCT detector is aligned. Pixel detector frozen.
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

19. Alignment accuracy vs number of hits
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

20. Combined TestBeam
Combined test beam in 2004 with all ATLAS subdetectors
Setup was like a slice from the ATLAS barrel
No beam telescope modules
Other tracking detectors provide a reference frame for SCT
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

21. Combined TestBeam - SCT residuals
Iteration 1
Iteration 30
Residuals of the 8 SCT modules in CTB without alignment corrections and with alignment corrections after 30 iterations
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

22. Combined TestBeam – alignment parameters
After a few iterations the alignment parameters of most degrees of freedom converge on stable values
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT

23. Conclusions - Outlook
Extensive beam tests with SCT modules were done to improve detector simulation and tune operating parameters of SCT
Performance and behavior of SCT modules understood well
Alignment algorithm for SCT is in place
Validation, testing and improvement of algorithm with simulated and test beam data is ongoing
SCT barrel is integrated with ATLAS drift tube detector (TRT) and takes data with cosmic rays
Important step towards final integration and commissioning
First alignment studies with cosmic data are being done
HLL Seminar Roland Härtel Integration and alignment of ATLAS SCT