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|