thijs cornelissen (infn genova ) on behalf of the atlas collaboration

Physics at LHC conference 7 June 2010, T.

Cornelissen 1

Thijs Cornelissen (INFN Genova)Thijs Cornelissen (INFN Genova)

On behalf of the ATLAS collaborationOn behalf of the ATLAS collaboration

Muon Muon

reconstruction reconstruction

performance in performance in

ATLASATLAS

Physics at LHC conference 7 June 2010, T. Cornelissen

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OutlineOutline

• The ATLAS detector

• Muon performance with cosmics

• Muon performance with collision events


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Immersed in a solenoid field of 2 Tesla measures the trajectories of charged particles.

The ID comprises 3 sub-detectors: (resolution)Pixel : 10/115 μm in Rϕ/zSilicon strip (SCT): 17/580 μmTransition radiation tracker (TRT): 130 μm in Rϕ

The ID covers : |η| < 2.5 (2.0 for TRT) with 3 Pixel measurements, 8 SCT and ~30 TRT.

Designed for tracking efficiency >90% () and 99% () , momentum measurement with σpT /pT = 0.05% pT ⊕1% and impact parameter (at high p) = 10 m

The Inner Detector (or inner The Inner Detector (or inner tracker)tracker)


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The muon spectrometerThe muon spectrometer

• Immersed in a toroidal magnetic field of ~ 0.5 T (3x8 superconducting toroids)

• Precision measurement chambers and trigger chambers– MDT (Monitored Drift Tubes)– RPC (Resistive Plate Ch.)– CSC (Cathode Strip Chambers)– TGC (Thin-Gap Chambers)

• coverage |η|<2.7


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Muons can be reconstructed in the inner detector (ID), the calorimeters, and/or

the muon spectrometer (MS)

Several different types of reconstruction algorithms:

‘Standalone’ muons are reconstructed in the MS, then extrapolated to the vertex

‘Combined’ muons are produced by matching an ID track to a MS track, then either a

full refit or a weighted average is performed

Gives best possible momentum resolution

‘Tagged’ muons are ID tracks matched with one or more segments in the MS

Useful in particular if the momentum is insufficient to leave a full track in the MS

Initial studies focused on efficiencies and resolutions, using ~0.6 nb-1 recorded

with the minimum bias trigger

Muon reconstruction algorithmsMuon reconstruction algorithms


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2008 and 2009: cosmics studies2008 and 2009: cosmics studies


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Inner detector cosmics studiesInner detector cosmics studies

• Track parameter resolution studied by splitting cosmic track into upper and lower half, and comparing the parameters

• Tracking efficiencies very high, as expected (muons)

• Cosmics probe mainly barrel region, statistics in endcaps much lower


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• Momentum resolution looks promising (better than ID for pT > 100 GeV)

• Resolution studies using combined tracks ongoing

Large sectors

Small sectorsMDT residualsnarrow=98 m

Muon spectrometer cosmics Muon spectrometer cosmics studiesstudies

ATLAS preliminary

ATLAS preliminary


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• Most particles in the 900 GeV run had low momentum, did not make it much further than the inner tracker.

• Still, O(100) muon candidates recorded (mostly from pion/kaon decays), and even a couple of di-muon events

Muons in 900 GeV runMuons in 900 GeV run


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Track parameter distributionsTrack parameter distributionscombined muons

Basic track parameter distributions show good agreement between data and simulationSimulation shows contributions from prompt muons (including b/c decays) in red, early /K decays in green, late decays in white


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Number of ID hits on track well described by simulation (see presentation by T. Kuhl)Good agreement between data and simulation concerning number of MDT and CSC hits on track

Number of hits on trackNumber of hits on track

combined muons

2 stations

3 stations

ATLAS preliminary ATLAS preliminary


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Reconstruction efficiencies estimated by

comparing overlap between algorithms

Use tagged muons that are confirmed as

calorimeter muons, check if combined

algorithm found the muon

Efficiencies underestimated due to

contamination of sample by /K decays

Decays are often rejected by combined

algorithms

Ultimately, efficiencies will be determined

using tag&probe with resonances

First efficiency measurementsFirst efficiency measurements

ATLAS preliminary


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Resolution of MS tracks studied by

comparing momentum with ID

track

Expected resolution ~5% for MS

tracks, ~2.5% for ID tracks (on

average)

Asymmetric tail due to /K decays,

larger in data than in simulation

Under investigation

Momentum resolutionMomentum resolution

ATLAS preliminary


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Muons inside jets have much

larger energy deposits around

them

Excellent data/MC agreement

concerning energy deposits

Simulation can thus be used to

optimize isolation cuts

Muon energy deposit and isolationMuon energy deposit and isolationTra

ck

isola

tion

Calo

rim

ete

r is

ola

tion

ATLAS preliminary

ATLAS preliminary

ATLAS preliminary


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• First 0.6 nb-1 has been used to assess the performance of the ATLAS muon reconstruction

• Relative efficiency measurements, muon isolation studies show very good data/MC agreement

• Now working towards full program using J/s and Zs

ConclusionsConclusions


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Backup


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Composition in MC truthComposition in MC truth

ATLAS preliminary


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7/4/2010

State of ATLAS for first 7 TeV State of ATLAS for first 7 TeV collisionscollisions

Muon EE chambers (||~1.2) not yet installed

thijs cornelissen (infn genova ) on behalf of the atlas collaboration

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