Chapter 2
The ATLAS experiment
Contents
2.1 Introduction ..................................................................................................................... 13
2.2 Detector set-up................................................................................................................. 13
2.2.1 Detector components .................................................................................................. 13
2.2.2 Particle identification .................................................................................................. 14
2.3 Magnet system ................................................................................................................. 15
2.4 Inner detector .................................................................................................................. 17
2.4.1 Introduction................................................................................................................. 17
2.4.2 Precision tracker.......................................................................................................... 20
2.4.3 TRT ............................................................................................................................. 24
2.4.4 Radiation environment................................................................................................ 26
2.4.5 Material ....................................................................................................................... 26
2.5 Calorimeter ...................................................................................................................... 27
2.6 Muon spectrometer ......................................................................................................... 28
2.6.1 Introduction................................................................................................................. 28
2.6.2 Muon precision system ............................................................................................... 29
2.6.3 Muon trigger detector system...................................................................................... 31
2.7 References ........................................................................................................................ 32
2.1 Introduction
This chapter describes the ATLAS experiment, which is one of the future LHC experiments.
It is a so-called general-purpose experiment, studying all physics topics described in section 1.4.
The global detector set-up is described in section 2.2. The ATLAS magnet system is described in section 2.3. Each detector component is described in a separate section: the inner
detector is described in section 2.4, the calorimeter is described in section 2.5 and the muon
spectrometer is finally described in section 2.6. The ATLAS trigger and data acquisition system will be described separately in chapter 5.
2.2 Detector set-up
2.2.1 Detector components
The ATLAS detector (figure 2.1) has a layout that is typical for a collider detector and consists of two types of detector components: tracking detectors, which measure the position of
a crossing charged particle with minimal disturbance, and calorimeters, which measure the
energy of a particle by total absorption. From the collision point outwards first tracking detectors (inner detector) are placed, then calorimeters, divided in electromagnetic (em) and
hadronic calorimeters, and then tracking detectors (muon spectrometer) again.
The complete ATLAS detector is split into a barrel part, where detector layers are positioned on cylindrical surfaces around the beam axis, and two end-cap parts, where detector
13
Chapter 2
The ATLAS experiment
layers are positioned in planes of constant z perpendicular to the beam pipe. The calorimeter
consists also of a forward1 and a backward part, extending up to a pseudorapidity of |η| = 4.9.
The most important dimensions of the ATLAS detector are summarised in table 2.1.
Each detector component is optimised to satisfy various requirements, e.g. resolution with
respect to position and/or energy, particle identification, covered range, costs and material.
The inner detector and muon spectrometer are placed in a magnetic field for the measurement of the momentum of charged particles. This magnetic field causes a bending of the
track, with a radius of curvature dependent on the momentum value.
ATLAS
Hadron
Calorimeters
Forward
Calorimeters
S. C. Solenoid
S. C. Air Core
Toroids
Muon
Detectors
Inner
Detector
EM Calorimeters
Figure 2.1 Three-dimensional view of the ATLAS detector.
Table 2.1 Dimensions of the ATLAS sub-detectors.
component
barrel muon spectrometer
end-cap muon spectrometer
barrel hadronic calorimeter
end-cap hadronic calorimeter
barrel em-calorimeter
end-cap em-calorimeter
forward/backward calorimeter
barrel + end-cap inner detector
radius [m]
length [m]
11
26
11
2.8
4.25
12.2
2.25
2.25
2.25
6.42
2.25
0.63
integrated in end-cap
1.15
6.8
η-coverage
|η| < 1.4
1.1 < |η| < 2.8
|η| < 1.0
1.5 < |η| < 3.2
|η| < 1.4
1.4 < |η| < 3.2
3.1 < |η| < 4.9
|η| < 2.4
2.2.2 Particle identification
Each particle gives a different signature in the ATLAS detector, making particle identification possible. The signatures for the most important particles are summarised in figure 2.2.
1
Throughout this thesis the term forward is also used for the end-cap part of the inner detector at positive z (opposite side backward).
14
2.3 Magnet system
An electron gives a signal in the inner detector, losing a small part of its energy (see also
section 2.4.5), and in the calorimeter (mainly in the electromagnetic calorimeter), depositing
its remaining energy part. Photons give also a signal in the calorimeter, but not in the inner
detector (unless a photon converts into an electron-positron pair). Muons traverse the calorimeter and give a signal in the inner detector and muon spectrometer and eventually a small
signal in the calorimeter. Charged hadrons give a signal in the inner detector and the calorimeters. Hadrons shower2 deeper into the calorimeter than electrons and photons and give a
signal in both the electromagnetic and hadronic calorimeter. Jets (not shown in figure 2.2) are
a combination of hadronic and leptonic particles, and cause a signal in the inner detector and
calorimeters and eventually also in the muon spectrometer. Neutrinos can not be detected and
leave only a signal of missing transverse energy, see also section 1.1.
Figure 2.2 Signature of some highly energetic particles in the inner detector (inner tracker), calorimeter and muon spectrometer (outer tracker).
2.3 Magnet system
An appropriate magnetic field distribution is required for measuring the transverse momenta
of the produced charged particles. The magnet system of the ATLAS detector consists of four
superconducting magnets:
• A central solenoid [1]
• An air-core barrel toroid [2]
• Two air-core end-cap toroids [3]
A three-dimensional view of the bare windings of the ATLAS magnet system is given in
figure 2.3. In principle the three toroidal magnets could have been combined into a single
large toroidal magnet. For technical reasons the toroidal system is split into three subsystems [4].
2
A shower is defined as the cascade production of electrons, photons and hadrons (for
hadron showers) initiated by a highly energetic particle that was incident on a thick absorber.
See also section 2.5.
15
Chapter 2
The ATLAS experiment
Toroid magnets
Each toroid consists of eight coils with 120 (barrel) or 116 (end-cap) turns. As all coils are
superconducting, cooling circuits, a vacuum system and cryostats for optimum thermal insulation of the coils are required. Each coil has an operation temperature of 4.5 K [4] and is enclosed by a cryostat. The Lorentz forces between the coils and the weight of the coils require
additional mechanical structures between the coils (“voussoirs” and “struts” [4]).
The toroid magnets generate a toroidal magnetic field configuration for the muon spectrometer. The advantage of a toroidal magnetic field is that its direction is almost perpendicular to the direction of flight of the particles.
Central solenoid
The central solenoid consists of one coil with 1173 turns. The central solenoid is designed to
provide for the inner detector an axial magnetic field of 2 T. Its axis coincides with the beam
axis. The axial length of the solenoid is 5.3 m. The magnetic field points in the positive
z-direction.
Field integrals
The most important numbers for track momentum measurements are the field integrals over
the track length inside the tracking volume [5]:
∫
I 2 = ∫∫ B sin θ (dl B ) dldr
I1 = B sin θ ( dlrBr ) dl
(2.1)
rr
with tanθ the slope in the (r, z) plane. I1 is the integral of Bdl as a measurement of the bending power of the field. I2 is the double integral of the field that is especially important for
momentum measurements (see chapter 3).
The toroidal magnetic field provides for typical bending powers of 3 Tm in the barrel and
6 Tm in the end-cap regions [4]. The solenoidal magnetic field provides for a typical bending
power of 2.1 Tm [4]. The tracking capacity of the ATLAS solenoid has been compared with
the tracking capacity of an ideal solenoid field [6]. The degradation in the tracking capacity in
the high rapidity region (1.6 < |η| < 3.2) is at most 10%.
Figure 2.3 Three-dimensional view of the bare windings of the ATLAS magnet system: the
central solenoid, the 8 coils of the barrel toroid and the 2 × 8 coils of the end-cap toroids.
16
2.4 Inner detector
2.4 Inner detector
2.4.1 Introduction
The inner detector is the part of the ATLAS detector placed the most close to the interaction
point. The environment inside the electromagnetic calorimeter is very hostile: everything in
the inner cavity is subject to a high flux of pions, photons and neutrons. The pions and photons are mostly created in the interaction point. The neutrons are due to backsplash from the
calorimeters.
A three-dimensional view of the inner detector is given in figure 2.4. The ATLAS inner
detector combines high-resolution detectors at inner radii with continuous tracking elements
at outer radii, all contained in a solenoid magnet with a central field of 2 T.
Requirements
The ATLAS inner detector has been developed to satisfy various physics requirements on
electron identification, photon identification, identification of decaying K S0 mesons and the
reconstruction of secondary vertices due to the decay of particles containing bottom quarks.
These requirements are described in more detail in the inner detector TDR (Technical Design
Report) [6]. They can be translated into requirements for the number of measurement points,
the resolution of the measurement points, the covered range in η and r and track3 reconstruction specifications. Also these requirements are described in more detail in the TDR [6]. The
momentum resolution requirement for a 500 GeV track varies between 30% and 50%, see
also chapter 3.
Detector set-up
Silicon pixel detectors with the highest granularity are placed closest to the interaction point.
Further away from the interaction point, silicon microstrip detectors (SCT, SemiConductor
Tracker) are placed. The total number of silicon precision layers must be limited because of
the material they introduce, and because of their high cost. In the current design of the
ATLAS detector, a particle travelling from the interaction point crosses at least four strip layers and three pixel layers. This is shown in figure 2.5, giving one hit for each pixel layer, and
two for each strip layer (rφ and stereo measurement, see section 2.4.2).
The straw tube tracker (TRT, Transition Radiation Tracker) gives a much larger number
of tracking points (typically 36 points per track), however with less accuracy, and provides
the possibility of continuous track following with much less material per point and at lower
cost. In figure 2.6 the number of crossed straws in the TRT is plotted.
The combination of the TRT and precision tracker makes a very robust track recognition
possible and provides a high precision in (φ, r, z) co-ordinates. The relative precisions of the
different measurements (pixel detectors, SCT, TRT) are well matched, so that no single
measurement dominates the momentum resolution. This is important for reasons of robustness, in the event that a single system does not perform to its full specification.
A cross-section of the engineering layout of the inner detector through the beam axis is
given in figure 2.7. The outer radius of the inner detector is 115 cm. This value is a compromise between the best possible tracking capacity (equation (2.1)) and restrictions due to the
inner dimension of the solenoid coil. The total length is 7 m, limited by the position of the
3
The trajectory of a charged particle through the magnetic field of the inner detector (or
muon spectrometer) is referred to as track.
17
Chapter 2
The ATLAS experiment
end-cap calorimetry. The precision trackers elements are contained within a radius of 56 cm,
followed by the TRT, and finally the general support and service area at the outermost radius.
Barrel SCT
Forward SCT
TRT
Pixel Detectors
Number TRT Hits
Number Hits
Figure 2.4 Three-dimensional view of the ATLAS inner detector.
SCT
Pixels
10
5
40
20
0
0
0
1
2
3
0
|η|
0.5
1
1.5
2
2.5
|η|
Figure 2.5 Number of hits per track in the pre- Figure 2.6 Number of hits per track in the
cision detectors. Taken from [6].
TRT. Taken from [6].
18
2.4 Inner detector
Figure 2.7 Cross-section of the inner detector layout through the beam axis.
19
Chapter 2
The ATLAS experiment
2.4.2 Precision tracker
The precision tracker consists of pixel and microstrip detectors, both based on silicon technology.
The pixel detector is designed to provide a set of very high-precision measurements as
close to the interaction point as possible. The system provides three of the precision measurements over the full acceptance, and determines the impact parameter resolution (see
chapter 3) and hence the ability of the inner detector to find short-lived particles such as bottom quarks. The two-dimensional segmentation of the sensors requires the use of advanced
electronic techniques and interconnections for the readout. The readout chips of the pixel
sensors are described in more detail in the TDR [7]. The total pixel system consists of
1.4 × 108 detector elements.
The SCT (silicon microstrip detector) is designed to provide at least four of the precision
measurements per track in the intermediate radial range, and contributes to the measurement
of transverse momentum, impact parameter and vertex position. The system is an order of
magnitude larger in surface than previous generations of silicon microstrip detectors, and in
addition must face much higher radiation levels. The total SCT contains 61 m2 of silicon detectors, with 6.2 × 106 readout channels.
Silicon detector operation
Silicon detectors are based on the release of free charge by a particle traversing matter. When
a diode is biased in the reverse direction, there is only a small (dark) current. On the passage
of a particle through the depleted region, electron-hole pairs are created, which results in a
briefly enhanced conductivity of the depletion region. This gives rise to a peak in the electric
current through the detector.
Barrel pixel detector
In the barrel area, the pixel layers are segmented in rφ and z. The pixel size is 50 µm in the
rφ-direction and 300 µm in the z-direction. The system consists of three layers at average radii of about 4 cm, 11 cm and 14 cm. The innermost barrel layer is especially useful for bottom quark physics. For this reason, this layer is also referred to as B-layer. In figure 2.7 this
layer is referred to as vertex barrel.
The B-layer will need replacement after a few years at high luminosity, because radiation
damage will limit the lifetime of this layer. The exact time depends on the luminosity profile.
It has not yet been decided whether the B-layer will be present during the whole lifetime of
the inner detector, or only at the initial lower luminosity running.
The barrel pixel system is very modular, containing approximately 1500 identical detector
modules. Only one type of support structures is used. Each barrel module is 62.4 mm long
and 22.4 mm wide, with about 6.1 × 105 pixel elements, readout by 16 chips each serving an
array of 24 by 160 pixels. The modules are overlapping in order to give hermetic coverage.
Simulations show that the thickness of each layer is less than 1.39% of a radiation length [6].
The most important characteristics of the barrel pixel system are summarised in table 2.2.
Table 2.2 Characteristics of barrel silicon pixel detector.
radius [mm]
47.5
105.5
137.5
ò length of
cylinder [mm]
387
387
387
tilt angle
[degrees]
10.5
9.5
9.5
row pitch
[µm]
50
50
50
20
column
pitch [µm]
300
300
300
number of
modules
260
572
754
2.4 Inner detector
End-cap pixel detector
The end-cap pixel system consists on each side of the interaction point of four disks with
modules, placed between radii of 11 and 20 cm. These disks are located perpendicular to the
beam axis, using identical support structures. The pixels are segmented in rφ and r. The pixel
size is 50 µm in the rφ-direction and 300 µm in the r-direction. The total end-cap consists of
1000 identical disk modules. The end-cap modules are very similar in design to the barrel
modules. The most important characteristics of the end-cap pixel system are given in
table 2.3.
Table 2.3 Characteristics of end-cap silicon pixel detector.
z
[mm]
490
608
759
1035
inner radius
[mm]
107.1
107.1
107.1
151.0
outer radius
[mm]
196.0
196.0
196.0
196.0
row pitch
[µm]
50
50
50
50
column
pitch [µm]
300
300
300
300
number of
modules
140
140
140
80
Barrel SCT
The barrel part of the SCT consists of four cylindrical layers of modules, placed at radii of
300, 373, 447 and 520 mm. Each module consists of four detectors. Each silicon detector is
6.36 cm × 6.40 cm with 768 readout strips of 80 µm pitch. On each side of a module, two
detectors are bound together to form 12.8 cm long strips, with a 2 mm dead area in the middle. Two such detector planes are glued together at a 40 mrad angle. This small stereo angle
is used to obtain the z-measurement of the (φ, r, z) precision points. The configuration of a
barrel SCT module is shown in figure 2.8, an expanded view is shown in figure 2.9. The silicon detectors are glued to a central beryllia (BeO) baseboard and a heat spreader made of
pyrolytic graphite (TPG [7]). An important reason to use beryllia is its long radiation length,
hence minimal disturbance of the track.
Modules are staggered in radius by ±1 mm to give overlap in z, and an overlap of 1% in φ
makes the detector hermetic for tracks with a transverse momentum of more than 1 GeV. The
most important characteristics of the barrel SCT are given in table 2.4. A transverse view of a
quadrant of the ATLAS barrel silicon layers is given in figure 2.10. The tilt angle made by
the modules and the staggered structure is clearly visible.
21
Chapter 2
The ATLAS experiment
Figure 2.8 Configuration of barrel SCT module.
Figure 2.9 Expanded view of a barrel module.
22
2.4 Inner detector
Table 2.4 Barrel SCT characteristics.
radius
ò length
tilt angle
[mm] of cylinder [mm] [degrees]
300.0
746.7
10.0
373.0
746.7
10.0
447.0
746.7
10.0
520.0
746.7
10.0
pitch
[µm]
80.0
80.0
80.0
80.0
strip length
[mm]
126
126
126
126
number of orientation4
modules
12 × 32
φ,u
12 × 40
φ,v
12 × 48
φ,u
12 × 56
φ,v
Figure 2.10 Transverse view of the ATLAS barrel precision layers (SCT + pixel detector).
End-cap SCT
The end-cap modules are very similar in construction but use tapered strips, with one set
aligned radially, and the other with a 40 mrad stereo angle α. End-cap modules are made in
two versions with lengths of about 12 and 7 cm. The 12 cm strips consist of two parts, with a
2 mm dead area in the middle. The layout of an end-cap module is shown in figure 2.11, an
expanded view is shown in figure 2.12. The end-cap modules are mounted in up to three
rings onto nine wheels, which are interconnected by a space frame. The most important characteristics of the end-cap part are given in table 2.5.
The “u-layer” makes a positive stereo angle α of +40 mrad. The “v-layer” makes an angle α
of –40 mrad with the z-direction. The “u/v-layer” is also referred to as “stereo-layer”. The
“φ-layer” is placed parallel to the z-direction.
23
4
Chapter 2
The ATLAS experiment
Figure 2.12 Expanded view of an end-cap
module.
Figure 2.11 End-cap module layout.
Table 2.5 End-cap SCT characteristics.
z
[mm]
radius (i/o)
[mm]
# modules
[i/m/o]
835
925
1072
1260
1460
1695
2135
2528
2788
259-560
336-560
259-560
259-560
259-560
259-560
336-560
401-560
440-560
40/40/52
-/40/52
40/40/52
40/40/52
40/40/52
40/40/52
-/40/52
-/40/52
-/-/52
strip length
(i/m/o)
[mm]
72/117/121
-/117/121
72/117/121
72/117/121
72/117/121
72/117/121
-/117/121
-/72/121
-/-/121
inner strip
pitch
(i/m/o) [µm]
54/70/71
-/70/71
54/70/71
54/70/71
54/70/71
54/70/71
-/70/71
-/54/71
-/-/71
outer strip
pitch
(i/m/o) [µm]
70/95/90
-/95/90
70/95/90
70/95/90
70/95/90
70/95/90
-/95/90
-/70/90
-/-/90
orientation
φ,u
φ,v
φ,u
φ,v
φ,u
φ,v
φ,u
φ,v
φ,u
2.4.3 TRT
The TRT (Transition Radiation Tracker) contributes to the transverse momentum measurement. The larger number of measurements and the higher average radius compensate the
lower precision per point of the TRT compared to the precision tracker. The TRT is not used
for impact parameter measurements (see chapter 3).
The TRT is based on the use of straw detectors, which can operate at the required very
high rates by virtue of their small diameter and the isolation of the sense wires within individual gas envelopes. This technique is intrinsically radiation hard, and allows a large number
24
2.4 Inner detector
of measurements to be made on every track at modest cost. However the detector must cope
with a large occupancy and high counting rate at the LHC design luminosity. At this luminosity, the measurement accuracy, averaged over all straws, is better than 50 µm, including
errors from alignment [7]. A schematic view of the TRT detector in the (r, z) plane, together
with the main dimensions is given in figure 2.13.
Straw tube operation
A straw tube is a thin cylindrical tube with a conducting inner surface at negative potential. A
wire is strung in its centre, which is held at positive voltage. The straws are filled with xenon
gas. The distance of a traversing particle from the anode wire can be derived from the arrival
time of the signal on the anode (drift time measurements). Electron identification capability
is added by conversion in the xenon gas of transition-radiation photons, created in thin
foils between the straws. The creation of transition-radiation photons is based on the following mechanism: when a charged particle with energy E and mass m crosses a transition of two
materials with a different dielectric constant, it has a probability proportional of γ = E/m to
emit photons in the keV range. This effect is most pronounced for electrons due to their high
γ-factor.
Each straw is 4 mm in diameter, giving a fast electrical response and good mechanical
properties for a maximum straw length of 150 cm. With each straw a measurement of the
drift time and amplitude is possible. The drift time measurement corresponds to a spatial
resolution of 170 µm per straw. The use of two independent amplitude thresholds allows the
detector to discriminate between hits without accompanying transition radiation hits, which
pass the lower threshold, and hits with accompanying transition radiation hits, which pass the
higher.
Barrel TRT
The barrel section is built of individual modules with between 329 and 793 axial straws each
(parallel to the beam direction), covering the radial range from 56 to 107 cm. The total number of straws in the barrel is about 5 × 104. Each straw is divided into two at the centre in order to reduce the occupancy and is read out at each end.
End-cap TRT
One end-cap part consists of 18 wheels with radial straws. The 14 wheels nearest to the interaction point cover the radial range from 64 to 130 cm. The last four wheels extend to an inner
radius of 48 cm. This is necessary to maintain a constant number of crossed straws over the
full acceptance. The wheels 7 to 14 have half as many straws per cm in z as the other wheels.
The total number of straws in the end-cap part is about 3.2 × 105. Each straw is readout at the
outer radius.
25
Chapter 2
The ATLAS experiment
Figure 2.13 Schematic view of the TRT detector in the (r, z) plane, together with the main
dimensions.
2.4.4 Radiation environment
The radiation levels in the inner detector cavity will be extremely high, leading to damage in
the silicon detectors, degradation of the electronics performance and contribution of background hits in the sensitive elements. The large |η| coverage required in the very small available volume makes it necessary to install the detector components close to the beam axis.
The most relevant quantity for the damage in silicon detectors is the fluence expressed in
terms of 1 MeV equivalent neutrons. The typical values vary between 1.5 × 1013 cm-2/year for
the SCT to 5 × 1013 cm-2/year for the pixel detector [6]. All materials used in the inner detector cavity must be qualified to survive the doses and fluxes expected at the positions at
which they are placed.
The precision tracker is operated at low temperature because the effect of silicon radiation
damage is strongly temperature dependent. As nominal operating temperature has been chosen -5 Û&WR-10 Û&-7 Û&DYHUDJHLQWKH6&7:LWKWKLVRSHUDWLQJWHPSHUDWXUHWKHSUHFLVLRQ
tracker will survive an operation of ten years at high luminosity. This low temperature also
has the beneficial effect of reducing the leakage current and hence heat-generation inside the
detector substrate. The entire silicon system will be enclosed in a cold envelope, with an active shield preventing heat transfer from the TRT.
All the electronics in the inner detector cavity will be purchased from vendors using recognised radiation-hard chip fabrication processes.
2.4.5 Material
Tracking detectors must cause the smallest possible disturbance of the tracks passing through
it. This means that the amount of material in the inner detector must be as minimal as possible. There are however several boundary conditions which cause that the amount of material
in the ATLAS detector will be much larger than that of previous tracking detectors. This is
mostly due to the necessity of using radiation hard components and the necessity of installing
all the front-end electronics components on the detector itself.
26
2.5 Calorimeter
Absorption length
Radiation length
The cumulative distributions for the number of radiation lengths for the pixel detector,
SCT, TRT and the services5 are shown in figure 2.14. The corresponding distributions for the
absorption length are given in figure 2.15. From these distributions it follows that the amount
of material in the inner detector is significant, especially in the transition region between barrel and end-cap. This large amount of material makes it much more difficult for the calorimeter to do a correct energy reconstruction. The consequences are described in the calorimeter performance TDR [8]. For the inner detector the consequences are also important:
• Tracks undergo significant multiple scattering
• There is a significant increase in multiplicity due to secondary interactions
• Electrons have a significant bremsstrahlung probability
• Photons have a significant probability to convert into an electron-positron pair
• Absorption of hadrons, causing tracks to be lost
The consequences for the inner detector are described in more detail in the inner detector
TDR [6]. The influence of multiple scattering on the detector resolution is described in more
detail in chapter 3.
1.4
Total
1.2
1
0.8
0.6
0.5
Total
0.4
0.3
0.2
TRT
TRT
0.4
SCT
0.1
SCT
Pixel
0.2
Pixel
0
0
0
0.5
1
1.5
2
2.5
3
3.5
|η|
Figure 2.14 Cumulative distribution for number of radiation lengths for (a) pixel detector,
(b) SCT, (c) TRT and (d) external services.
Taken from [6].
0
0.5
1
1.5
2
2.5
3
3.5
|η|
Figure 2.15 Cumulative distribution for number of absorption lengths for (a) pixel detector,
(b) SCT, (c) TRT and (d) external services.
Taken from [6].
2.5 Calorimeter
The calorimeters are placed between the inner detector and the muon spectrometer. The primary goal of the calorimeters is the energy measurement of electrons, photons and jets, and
the measurement of the missing transverse energy. The calorimeters however also provide
position and angular measurements and particle identification. Their radiation resistance must
allow operation for more than ten years of data-taken at high luminosity.
The principle of calorimetry is the energy measurement of an incident particle by total absorption, where a fraction of the total energy is transformed into a measurable quantity
(charge or light). An incident electron or photon gives rise to an electromagnetic shower that
5
Detector elements like cables, cooling pipes, support structures etc. are referred to as services.
27
Chapter 2
The ATLAS experiment
can be described by a cascade of e± and γ production (mainly bremsstrahlung and the creation
of e+e- pairs). An incident hadron gives rise to a hadronic shower consisting of an electromagnetic component (e±, γ), a hadronic component of strongly interacting particles and a
component of low energetic particles that are not detected. Particle identification is performed using both the electromagnetic and hadronic calorimeters on the basis of transversal
and longitudinal shower profiles. An electromagnetic shower gives mainly a signal in the first
part of the calorimeter (electromagnetic calorimeter). A hadron gives a signal in both parts of
the calorimeter.
Because of the special interest in photons and electrons the resolution of the electromagnetic calorimeter is of prime importance. The hadron calorimeters are less accurate, which is
also partly due to the nature of hadronic showers in the calorimeter. The design goal energy
resolution for photons and electrons is [8]:
σ E 0.1
0 .3
=
⊕ 0.01 ⊕
E
E
E
(2.2)
with E in GeV. The design goal energy resolution for hadrons is:
σ E 0 .5
=
⊕ 0.03
E
E
(2.3)
All calorimeters consist of three parts, a barrel part, an end-cap part and a forward/backward part. The forward/backward calorimeter extends to |η| = 4.9. This is needed to
identify events with missing transverse energy, e.g. SUSY events (chapter 4). Both the
hadronic and electromagnetic forward calorimeters are liquid argon based and are integrated
in the cryostats of the end-cap calorimeters. The barrel and extended barrel region of the
hadron calorimetry use iron plates with scintillation plates. In the end-cap-region the hadron
calorimeter is based on liquid argon. All electromagnetic calorimetry is based on liquid argon
and lead absorbers [9, 10].
To make position measurements possible, the calorimeters are segmented in cells. The
electromagnetic calorimeter uses a segmentation varying between ∆η × ∆φ = 0.003 × 0.1 and
∆η × ∆φ = 0.025 × 0.025. The hadronic calorimeter uses a coarser segmentation of
∆η × ∆φ = 0.1 × 0.1.
Other particles than primary muons that are not stopped in the calorimeter give rise to a
background signal in the muon spectrometer. The thickness of the electromagnetic calorimeter is about 25-30 radiation lengths. The thickness of the hadronic calorimeter is about 10 absorption lengths [11].
2.6 Muon spectrometer
2.6.1 Introduction
The outermost part of the ATLAS detector is a muon spectrometer. The task of the muon
system is to reconstruct the momentum and direction of flight of muon tracks with the highest possible resolution. The other particles (except the neutrinos and possibly the SUSY LSP
(chapter 4)) are already stopped in the calorimeters.
Primary particles that are not stopped in the calorimeter and penetrate into the muon spectrometer give rise to a background signal (primary background). Another source of backgrounds are neutrons and photons in the MeV range, produced by secondary interactions in
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2.6 Muon spectrometer
the calorimeters, shielding material, the beam pipe and machine elements (radiation background).
The muon detector system has approximately 1.3 × 106 readout channels (including the
trigger chambers, see below). As described in section 2.3, the muon spectrometer is situated
in a toroidal magnetic field with a typical bending power of 3 Tm for the barrel and 6 Tm for
the end-cap. The momentum resolution for 1 TeV muons varies between 10% and 20%
(chapter 3).
2.6.2 Muon precision system
The muon spectrometer is divided in a barrel and two end-cap parts. The barrel extends up to
a pseudorapidity of |η| ≈ 1. The end-cap chambers cover the pseudorapidity range of
|η| = 1 - 2.7. A side view of one quadrant of the muon spectrometer is given in figure 2.16.
Figure 2.16 Cross-section of one quadrant of the muon spectrometer.
Barrel
In the barrel region, cylindrical layers around the beam axis are used. A transverse view of
the barrel part of the muon spectrometer is given in figure 2.17. In the transverse plane, the
muon detector is divided in eight towers with large detection stations and eight towers with
small stations. A tower contains in general three layers of measurement stations to locate the
muon so that the curvature of its trajectory in the magnetic field can be determined. This results in knowledge of both direction and momentum of the particle. Due to the presence of
support structures, two small towers contain only two detection layers.
Each measurement station consist of two separated half stations. Each half station consists
of three or four layers of detection elements. The detection elements are monitored drift
tubes, described below. Optical alignment systems have been designed to meet the stringent
requirements on the mechanical accuracy and the survey of the precision chambers.
End-cap
Each end-cap part consists of four rings with detectors, concentric with the beam axis, at
about 7.5, 10, 14 and 21-23 m distance from the interaction point. Monitored drift tubes are
used for the precision measurement in the end-caps over most of the area. At large pseudora29
Chapter 2
The ATLAS experiment
pidity (|η| = 2-2.7), cathode strip chambers are used due to the demanding rate and background conditions.
The chamber planes are orthogonal to the beam axis with the drift tubes oriented in the
azimuthal direction. Viewed along the beam pipe the chambers are of trapezoidal shape. They
are arranged with the same 16-fold azimuthal segmentation as in the barrel, with large and
small chambers covering the same azimuthal range as the barrel chambers (figure 2.18).
ATLAS
Muon Spectrometer
Trigger chambers
Precision chambers
End-cap
toroid
Barrel
coils
Figure 2.17 Transverse view of the ATLAS muon spectrometer in the underground hall.
Figure 2.18 The end-cap wheel placed at 14 m with the MDT chambers arranged in 16 sectors with small and large chambers alternating. The barrel chamber towers are indicated.
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2.6 Muon spectrometer
MDT operation
Pressurised MDT (Monitored Drift Tube) chambers combine high intrinsic spatial resolution
with an internal monitoring system to observe internal deformations of the chamber. They
provide for a robust, cost-effective instrumentation suitable for mass production. The basic
detection element of an MDT is a cylindrical aluminium drift tube of 30 mm diameter and a
central wire of 50 µm diameter at 3270 V with respect to the tube [12]. The detector is operated with a non-flammable gas mixture at 3 bar absolute pressure for reduced diffusion and
ionisation fluctuation [12]. There is a linear relation between the drift time and drift distance.
A schematic drawing of a barrel MDT chamber is given in figure 2.19. The chambers for
the end-cap are of trapezoidal shape, but are of similar design otherwise. The average single
tube resolution has a value of 80 µm [12].
Figure 2.19 Schematic drawing of a rectangular MDT chamber constructed from
multilayers of three monolayers each, for installation in the barrel spectrometer.
CSC operation
CSCs (Cathode Strip Chambers) are multiwire proportional chambers with wires on a high
voltage strung parallel in a gas volume, closed by conducting planes at 0 V. One of the two
enclosing planes is appropriately segmented in strips with a readout pitch of 5 mm. The CSCs
have a symmetric cell in which the anode-conducting plane distance equals the anode wire
spacing. The anode wire spacing has now been fixed at 2.54 mm, which is considerably lower
than the tube radius of the MDTs to reduce the occupancy per wire. Precise positionmeasurements along the wires are achieved by determining the centre of gravity of the charge
induced on the strips of one of the two conducting planes. With prototypes, resolutions of
better than 50 µm have been achieved in test beams [12].
2.6.3 Muon trigger detector system
The muon spectrometer plays an important role in the ATLAS trigger system (chapter 5). The
muon trigger system covers the pseudorapidity range |η| ≤ 2.4. The muon trigger system must
allow for the online reconstruction of muon tracks above 6 GeV and 20 GeV (transverse
momentum) at the bunch-crossing frequency of 40 MHz.
The muon trigger detector system consists of dedicated and fast chambers especially developed for trigger purposes, which are independent from the MDT/CSC chambers and have
a low occupancy. The MDTs and CSCs can not be used in the trigger system because these
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The ATLAS experiment
chambers will have too long drift times, which can be much longer than the LHC bunchcrossing period of 25 ns.
A system of RPCs (Resistive Plate Chambers) provide trigger signals in the barrel region.
The trigger detector in the barrel is made up of three stations, each with two detection layers.
They are located on both sides of the middle MDT station, and either directly above or directly below the outer MDT station. The two stations near the centre provide the low-pT trigger (pT > 6 GeV). The third station, at the outer radius of the magnet, allows to increase the pT
threshold to 20 GeV, for the high-pT trigger. An RPC is a gaseous parallel-plate detector with
a typical spatial resolution of the order of 1 cm and a typical time resolution of the order of
1 ns. A detailed description of the RPCs is given in the muon spectrometer TDR [12].
TGCs (Thin Gap Chambers) provide trigger capabilities in the end-cap region. Seven layers of TGCs complement the middle MDT station. Two layers of TGCs complement the inner MDT station. TGCs are standard multiwire proportional chambers [12], but with small
anode-to-anode, (wire-to-wire) distance (1.8 mm) and small cathode-to-anode distance
(1.4 mm). The spatial and time resolution of TGCs is similar to RPCs.
2.7 References
1. ATLAS Magnet Project Collaboration, ATLAS Central Solenoid Technical Design Report, ATLAS TDR-9, CERN/LHCC 97-21 (1997).
2. ATLAS Magnet Project Collaboration, ATLAS Barrel Toroid Technical Design Report,
ATLAS TDR-7, CERN/LHCC 97-19 (1997).
3. ATLAS Magnet Project Collaboration, ATLAS End-cap Toroids Technical Design Report, ATLAS TDR-8, CERN/LHCC 97-20 (1997).
4. ATLAS Magnet Project Collaboration, ATLAS Magnet System Technical Design Report,
ATLAS TDR-6, CERN/LHCC 97-18 (1997).
5. V.I. Klyukhin, A. Poppleton and J. Schmitz, Field Integrals for the ATLAS Tracking Volume, ATLAS INDET-NO-023 (1993).
6. ATLAS Inner Detector Community, ATLAS Inner Detector Technical Design Report
Volume 1, ATLAS TDR-4, CERN/LHCC 97-16 (1997).
7. ATLAS Inner Detector Community, ATLAS Inner Detector Technical Design Report
Volume 2, ATLAS TDR-5, CERN/LHCC 97-17 (1997).
8. ATLAS Calorimeter Community, ATLAS Calorimeter Performance Technical Design
Report, CERN/LHCC 94-40 (1996).
9. ATLAS Calorimeter Community, ATLAS Tile Calorimeter Technical Design Report,
ATLAS TDR-3, CERN/LHCC 96-42 (1996).
10. ATLAS Calorimeter Community, ATLAS Liquid Argon Calorimeter Technical Design
Report, ATLAS TDR-2, CERN/LHCC 96-41 (1996).
11. ATLAS Collaboration, ATLAS Technical Proposal for a General Purpose pp Experiment
at the Large Hadron Collider at CERN, CERN/LHCC 94-43, LHCC-P2 (1994).
12. ATLAS Muon Collaboration, ATLAS Muon Spectrometer Technical Design Report,
ATLAS TDR-10, CERN/LHCC 97-22 (1997).
13. D. Fourier and L. Serin, Notes on Lecture Series Experimental Techniques.
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