muons in atlas

Muons in ATLASAlexander Oh

University of Manchesteron behalf of the ATLAS Collaboration

ATL-COM-MUON-2010-024

Alexander Oh (Manchester University) - ATLAS Collaboration 2

Muons

• Detector and Reconstruction• Alignment and Calibration status• Trigger Performance• Reconstruction Performance

4.10.2010


Detector: Muon Spectrometer

4.10.2010

• Fast trigger chambers: – Resistive Plate Chambers (barrel)– Thin Gap Chambers (end-caps)

• Precision chambers:– Monitored Drift Tubes (barrel

and end-cap)– Cathode Strip Chambers

(forward region 2.0 < eta < 2.7)• Toroidal magnetic field of 0.5 T

– barrel and two end-cap toroids– 8 fold geometry– complex field


Detector: Inner Detector

4.10.2010

• Silicon Tracker– Pixels (3 layers)– Strips (4 barrel layers, 9 end-cap layers)– Coverage |eta| < 2.5

• Transition Radiation Tracker– Coverage |eta| < 2.0

• Solenoidal magnetic field of 2.0 T


Muon Reconstruction

• Muon Spectrometer Standalone (MS) reconstruction

• MS and Inner Detector (ID) combined fit– Outside-in– Inside-out– Statistical combination

• Taggers– ID track is matched to MS

segments– ID track is matched to a MIP in

calorimeter

4.10.2010

Objective: Muon identification in a wide momentum range from GeV (Onia) to TeV (BSM).Make use of all detector components and use different strategies:


Alignment and Calibration Status

4.10.2010


Alignment and Calibration Status: MS

• Alignment– Optical alignment system

• Barrel: relative• End-caps: absolute

– Data with toroid B-field off.– Measure alignment with straight

tracks.

4.10.2010

Cosmics data 2008

barrel alignment±50 mm (large sectors)±100 mm (small sectors)

Cosmics data 2008

end-cap alignment±45 mm


Alignment and Calibration Status: ID

• Track based alignment– First alignment with cosmics tracks.– First collisions at 900 GeV were used

to improve the alignment.– Performance study with 7 GeV data.

New alignment constant set in progress.

4.10.2010


Trigger Performance

4.10.2010


Trigger• L1 custom hardware

– Require coincidence in eta phi view.– pT estimation based on geometrical

roads.– Road definition and time alignment

commissioned.

• HLT software– L2: Pattern recognition, LUT based pT

estimate – EF: Full muon reconstruction available

• L1, L2 and EF are commissioned and used for active rejection.

4.10.2010

see also Ricardo Goncalo: Atlas Trigger

TGC, timing optimization


Trigger• Efficiency determined by looking at

– Inclusive muons relative to offline. – Tag & Probe method on

J/Psi and Z events.• Trigger Efficiencies at L1

– barrel 75% – end-cap 95%

• Rel. Trigger efficiencies at HLT– L2 >98%– EF >98%

4.10.2010

L1 efficiency, T&P with J/PsiBarrel + End-caps

L1 efficiency, inclusive muonBarrel

EF efficiency, inclusive muonBarrel + End-caps


Trigger• Muon rates measured in data.• Extrapolation to higher luminosity.• Determines evolution of trigger strategy.

4.10.2010

Allocated rate


Reconstruction Performance

4.10.2010


Reconstruction Efficiency• Determination from

Cosmics– Select events with ID

tracks traversing MS barrel.

– Look for MS track in • top hemisphere

(pID > 5 GeV) • bottom hemisphere

(pID > 9 GeV)

– Measures reconstruction efficiency of MS conditional to a L1 trigger.

4.10.2010

Turn on curve:Good agreement between Data and MC

Average efficiency:97.4 ±0.1 %95.2 ±0.1 %

ATLAS Preliminary2009 Cosmic Data

ATLAS Preliminary2009 Cosmic Data


Reconstruction Efficiency• Relative Efficiency from Data

– Use independent algorithms to assess reconstruction efficiency.

– Combined algorithm efficiency estimated by comparing overlap to calorimeter and segment tag algorithms.

– Tag and Confirm method with sample of J/Psi’s (purity ~98%)

– Tag and Probe studies are under preparation with J/Psi and Z.

4.10.2010

rel. eff. (%) data rel. eff. (%) MC

pT > 4 GeV 95.7±1.1 93.8±0.2

pT > 6 GeV 98.7±0.9 97.4±0.2


(Non-)Prompt Muons

Muons from light hadrons (“non-prompt”)•Decay within the ID volume• Hadron and the secondary muon are reconstructed

as one track•momentum mismatch of ID and MS pt

Muons from heavy flavour (“prompt”)•Secondary muon produced close to IP•ID and MS measure the muon track.

Discriminate by “loss momentum” fraction between MS and ID pT measurement:

Use templates derived from MC to determine fractions of “prompt” and “non-prompt” secondary muons.

4.10.2010

K0s -> p+p-J/Y -> m+m-

ID barrel


(Non-)Prompt Muons

• Fraction of prompt muons determined in bins of pT and eta.• MC and data agree within uncertainties• Dominant systematic uncertainties:

– template shape at low pT

– MC flavor content at high pT

4.10.2010

ID barrel

ID end-cap


Momentum Scale and Resolution• Use MS and ID pT

measurement to extract scale and resolution.

• Disentangle “prompt” and “non-prompt” contribution.– Background template

from QCD MC– Signal template: Gauss ×

Landau

4.10.2010

extract scale (xG) and resolution (sG+sL)


Momentum Scale and Resolution

4.10.2010

End-cap region:Due to alignment resolution in data worse then in MC.

Barrel region:Generally good agreement between data and MC.


Summary & Outlook• Performance of detector,

trigger and reconstruction studied with first data and cosmic events.

• Good agreement between data and MC.

• Presented results used only a fraction of the available data.

• Many more results to come soon with the full data set!

4.10.2010


Backup

4.10.2010


Alignment and Calibration StatusMonitored Drift Tubes (MDT): Timing

• Constant time offset t0 for each channel.– Leading edge of drift

time spectrum.

• Radius – drift time relation (rt) determined from auto-calibration method– Adjusted using residual

distribution of track fit

• More statistics needed for final calibration– t0 from track segment

fitter– Resolution 20-50% worse

than optimal

4.10.2010

− t0 refit− RPC timing− testbeam


Fake Muons from pions

• Determine fraction of p being reconstructed as m.– decay-in-flight– punch-through

• High purity p sample from K0

s -> p+p-

– common vertex– transverse decay length [5 mm ,120 mm ]– p(p+p-) aligned with K0

s flight direction– Purity ~95%

• Probe: p ID track– p > 3 GeV– pT > 1 GeV

• Tag: Muon candidate associated to the p ID track.

4.10.2010

identified m


Fake Muons from pions• Fraction of fake muons extracted

by fit to K0s mass spectrum.

• Fake fraction – 0.101±0.014%

(combined and tagged)– 0.070±0.012% (combined only)

4.10.2010

• Good agreement between data and MC

muons in atlas

Documents

atlas muons

muon spectrometer barrel

barrel layers

rpc chambers

atlas rpc chamberis

barrel alignment50

krpc trigger barrel

half barrel