1. Muon-raying the ATLAS Detector
Rosemarie Aben
Supervisor: Peter Kluit
Track: Particle and Astroparticle Physics
Amsterdam Master of Physics Symposium, 16 April 2010

2. Outline LHC and ATLAS The muon spectrometer of ATLAS
Precision measurements
Sagitta Analysis
Sagitta Resolutions
ATLAS Now

3. Detection of Higgs decay: H  ZZ  4µ “Golden Channel”
Standard Model & Higgs
Solution: ?
Energy per Collision
14 TeV
Protons per bunch
1011
Bunches per ring
2835
Design Luminosity
1034 cm-2 s-1
Circumference
27 Km
Collisions per bunch 25
Detection of Higgs decay:
H  ZZ  4µ “Golden Channel”

4. Exotic physics and ATLAS

5. Layout of the Muon spectrometer
Trigger chambers: trigger on muons
Precision chambers: Reconstruction of muon tracks
Small and Large muon stations
Three layers of muon stations
Sector, containing 12
Measurement towers

6. Monitored Drift Tube (MDT) Chambers
Tube diameter
30 mm
Diameter Sense wire
50 μm
# Tubes per chamber
Up to 432
Length MDTs
1-6 m
# MDT chambers
1150

7. Precision Measurements and Sagitta
Needed accuracy MDT chamber: 10% for 1 TeV muons 
Knowledge about the precision of the chambers is extremely important.
Contributions to precision: alignment and calibration.
Reconstructed Track
Sagitta
Misalignment
Outer
Middle
Inner
Sagitta: measure for the precision of the MDT chambers.

8. Sagitta Distribution
Distribution of the sagittas for all barrel sectors.
# Tracks 
Sagitta [mm] 

9. Sagitta Distribution Improvements
Old alignment
Improved alignment
Misalignment was seen in certain towers: shifts and extra peaks.
With this knowledge improvements have been made.
New analysis shows good alignment!

10. Sagitta Resolution Fit
Resolution: necessary to monitor the performance of the muon chambers in measuring momentum and position.
Sagitta resolution is a measure for the accuracy of the chambers depending on alignment + calibration + chamber geometry + chamber deformations+ ...
10-20 GeV GeV GeV GeV
Slices up to 350 GeV.
Each slice is fitted with a double Gauss fit and the standard deviation of the smaller Gauss is taken to do the final fit, using:
σ (sagitta) = √( P1² + P2²/momentum²)
P1 = intrinsic resolution term: gives the sagitta resolution
P2 = multiple scattering term

11.  Look to simulated cosmic muons!
Sagitta Resolution from Cosmic Data
Latest results
Large sectors
Small sectors
Intrinsic term
Multiple Scattering term
Sagitta resolution: 101 ± 6 μm
Sagitta resolution: 143 ± 16 μm
Last November the sagitta resolutions were still around 200 μm….
But, is this as expected?
 Look to simulated cosmic muons!

12. Muons ready to be reconstructed in ATLAS!
Sagitta Resolution from Simulated Cosmics
Simulation
Large sectors
Small sectors
Sagitta resolution: 63 ± 2 μm
Sagitta resolution: 102 ± 5 μm
Comparison with data: 101 μm for Large sectors and 153 μm for Small sectors.
Difference between data and simulation due to effects not yet corrected for.
But accuracy is already good enough for the muons coming from latest collisions.
Muons ready to be reconstructed in ATLAS!

15. Time for questions.

16. Summary and Conclusions
BACK UP
The sagitta is a good measure to study the achieved precision of the MDT chambers with respect to alignment and calibration;
The sagitta resolution is a measure for the accuracy of the chambers;
Sagitta resolution:
- Resolutions for large sectors, data vs. MC: 101 μm vs. 63 μm.
- Resolutions for small sectors, data vs. MC: 143 μm vs. 102 μm.
The MDT chambers are ready for analysis of the 7 TeV Data!!!