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Rosemarie AbenSupervisor: Peter Kluit

Track: Particle and Astroparticle Physics

Muon-raying the ATLAS Detector

Amsterdam Master of Physics Symposium, 16 April 2010

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Outline

• LHC and ATLAS• The muon spectrometer of ATLAS• Precision measurements• Sagitta Analysis• Sagitta Resolutions• ATLAS Now

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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”

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Exotic physics and ATLAS

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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

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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

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Precision Measurements and SagittaNeeded 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

Sagitta: measure for the precision of the MDT chambers.

Outer

Middle

Inner

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Sagitta DistributionDistribution of the sagittas for all barrel sectors.

Sagitta [mm]

# Tr

acks

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Sagitta Distribution Improvements

Misalignment was seen in certain towers: shifts and extra peaks.

With this knowledge improvements have been made.

New analysis shows good alignment!

Old alignment Improved alignment

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Sagitta Resolution FitResolution: 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+ ...

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

10-20 GeV 20-40 GeV 40-70 GeV 70-110 GeVSlices up to 350 GeV.

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Sagitta Resolution from Cosmic Data

Large sectors

Sagitta resolution: 101 ± 6 μm

Intrinsic termMultiple Scattering term

Sagitta resolution: 143 ± 16 μm

Small sectors

Last November the sagitta resolutions were still around 200 μm….

But, is this as expected?

Look to simulated cosmic muons!

Latest results

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Sagitta Resolution from Simulated Cosmics

Sagitta resolution: 63 ± 2 μm

Large sectors

Sagitta resolution: 102 ± 5 μm

Small sectorsSimulation

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!

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Time for questions.

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Summary and Conclusions

• 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!!!

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