Study of the performance of the ATLAS Muon Spectrometer

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Study of the performance of the ATLAS Muon Spectrometer

ATL-MUON-PROC-2011-008

15 November 2011

Study of the performance of the ATLAS Muon

Spectrometer

Abstract—The ATLAS muon spectrometer consists of three

large air-core superconducting toroids and of systems of both

trigger and precise position measurement chambers of various

technologies. The aim of the spectrometer is to provide a

measurement of the muon transverse momenta with a resolution

below 10% from 3 GeV up to 1 TeV. The system is also designed

to trigger on muons in a wide angular region. The precision

tracking chambers are Monitored Drift Tube chambers (MDT)

covering most of the detector, and the Cathode Strip Chambers

(CSC) are located in a small part of the forward region. Resistive

Plate Chambers (RPC) and Thin Gap Chambers (TGC) are used

for triggering. The performance of the spectrometer in terms

of trigger and track reconstruction efficiency and in terms of

resolution are continuously measured using collision data. The

methods developed to assess the spectrometer performance will

be presented and discussed.

I. INTRODUCTION

DETECTION of muons is a crucial issue for the LHC

experiments. Muons are clean signatures of Standard

Model processes like single and double vector meson production,

and, at the same time, are important probes in the search

for phenomena beyond the Standard Model [1].

In LHC experiments, high momentum muons are filtered

by the calorimeters, so that tracking detectors placed outside

the calorimeters tag muons with high efficiency and rejection

capability. Tracking in magnetic field allows to measure the

muon momentum and extrapolate its direction to the inner part

of the detector in order to match it to a track reconstructed in

the Inner Detector.

The ATLAS Muon Spectrometer [2] (MS in the following)

is based on three large air-core superconducting toroids

providing a magnetic field integral ranging between 2 and

8 Tm, aiming to detect all muons with transverse momenta

pT between few GeV and few TeV produced within an

angular region extended up to |η|=2.71 . The design momentum

resolution is expected to be well below 10% up to 1 TeV.

In the following we describe first the Muon Spectrometer,

then we present the performance obtained using the collision

data collected in the first two years of LHC operation.

II. THE ATLAS MUON SPECTROMETER

Fig.1 shows the lay-out of the ATLAS Muon Spectrometer.

It consists of a Barrel with a cylindrical shape around the

beam-line and two Endcaps. The Barrel covers the pseudorapidity

region |η|


Fig. 2. Design momentum resolution of the ATLAS muon system for muons

in the Barrel (|η|


Efficiency

1

0.95

0.9

0.85

0.8

0.75

0.7

ATLAS Preliminary

Tag and probe, Data 2010, Chain 2


-1

Ldt = 42 pb

MC signal only

Data corrected

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

η

Fig. 5. Muon reconstruction efficiency as a function of η for combined

and tagged muons with pT larger than 20 GeV. Data (black filled points) are

compared to Montecarlo expectations (blu triangles).A correction to the data

is applied to take into account the small amount of residual background.

Efficiency

1.1

1.05

1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

s = 7 TeV

­1

∫ L dt = 3.1 pb

ATLAS Preliminary

CB or ST

data

MC

0.6

0 2 4 6 8 10 12 14 16 18 20

p [GeV]

T

Fig. 6. Muon reconstruction efficiency as a function of pT for combined

and tagged muons with |η| 6 GeV

T

s=

7 TeV

0.2

CB+ST MC Chain 2

­1

∫ Ldt = 3.1 pb

CB+ST Data Chain 2

0

­3 ­2 ­1 0 1 2 3

q × η

Fig. 7. Muon reconstruction efficiency as a function of q×η for combined

and tagged muons with pT larger than 6 GeV. Data (blue filled points) are

compared to Montecarlo (black empty points) expectations.

and 9 for the single muon trigger with a threshold of 18 GeV.

The efficiency at the plateau is about 80%. This is due to the

limited acceptance of some barrel regions, as can be seen by

the η dependence.

The agreement between data and Montecarlo simulation is

very good in all cases, apart from the trigger, where few%

differences are observed. Data/MC scale factors are evaluated

for each data period in order to correct the simulations for the

physics analyses.

EF_mu18 Efficiency

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

ATLAS Preliminary

­1

2011 Data ∫ L dt = 138 pb

DATA

MC

20 30 40 50 60 70 80 90 100

p [GeV]

T

Fig. 8. Muon trigger efficiency with respect to reconstruction, as a function

of pT for a trigger threshold of 18 GeV. Data (open circles) is compared to

Montecarlo (filled triangles).

EF_mu18 Efficiency

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

ATLAS Preliminary

­1

2011 Data ∫ L dt = 138 pb

DATA

MC

­2 ­1.5 ­1 ­0.5 0 0.5 1 1.5 2

Fig. 9. Muon trigger efficiency with respect to reconstruction, as a function

of η for a trigger threshold of 18 GeV. Data (open circles) is compared to

Montecarlo (filled triangles).

B. Momentum resolution

The momentum resolution of the Muon Spectrometer is

parametrized according to the following formula:

σ(pT )

pT

= pMS 0

pT

⊕ p MS

1

⊕ p MS

2

× pT

It is the sum of three terms with three corresponding parameters:

the first term is due to the energy loss fluctuations in

the calorimeter (pMS 0 ), the second depends on the multiple

scattering (pMS 1 ) and the third is related to the intrinsic hit

resolution that in turn depends on alignment and calibration

(p MS

2 ) .

The values of the three parameters have been obtained by

a fitting procedure applied to the data taken in 2010. The

parameters have been adjusted to fit the Z line shape and

to account for the distributions of the differences between

momenta measured in MS and ID for muons coming from

the W meson decay. Fig.10 shows the parametrization of the

MS resolution obtained for muons in the Barrel in the pT range

between 20 and 100 GeV extrapolated up to 200 GeV. It can

be seen that the resolution in the 2010 data was significantly

above the simulation values, in most of the momentum range.

This is due essentially to two effects: first, in the data shown

here, alignment and calibration were not in final shape; second,

the Montecarlo simulation didn’t account for all the material

present in the actual set-up and for all the deformations in

the actual MS geometry. A new analysis on 2011 data with

improved alignment and calibrations is in progress together

η

3


T

)/p

σ(p

T

0.22

0.2

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

s=

7 TeV


­1

L = 40 pb

ATLAS Preliminary

Barrel MS ( | η|

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