The coordinate system widely used to describe the ATLAS detector is as follows. The origin of the coordinate system is defined by the nominal interaction point. A right-handed coordinate system is used, with the z-axis defined by the beam direction.
Side-A of the detector represents the positivez direction. Thex−yplane is orthogonal
to the beam direction, where the positive x-axis direction points from the interaction
point to the centre of the LHC ring, and the positive y-axis direction points upwards
along the zenith. The azimuthal angle ϕ is measured around the beam axis, and the
polar angle θ is measured from thez axis. The pseudorapidity η is defined as
η=−ln tan (θ/2), (3.1)
which is equal to rapidity when a physics object is approximately massless. For the case where there is an appreciable mass, for instance jets, rapidityy is more appropriate,
y= 1 2ln E+pz E−pz ! , (3.2)
where E is the energy and pz is the momentum projection in the z-direction of the physics object. The distance ∆R in pseudorapidity-azimuthal space is defined as
∆R=q∆ϕ2+ ∆η2, (3.3)
where ∆ϕ and ∆η are defined as the separation in azimuthal and pseudorapidity space,
respectively.
3.3
The Inner Detector
The inner detector (ID) is located closest to the beam pipe and the interaction point (IP), providing high-precision momentum measurements of charged particles and also vertex identification, with a fine detector granularity needed for the large track density. The ID consists of the pixel and silicon microstrip (SCT) trackers, which are surrounded by the straw tubes of the transition radiation tracker (TRT). The layout of the various sub-components during Run 1 are shown in Figure 3.4. During Long Shut Down 1 (LS1), the ID was upgraded to include an additional pixel layer closest to the beam pipe, known as the Insertable B-layer (IBL) [78], compensating for the eventual deterioration
3.3 The Inner Detector 35
of the current pixel layers from radiation effects. This also provides the benefit of improved vertex resolution, particularly of secondary vertices necessary for B-physics
studies. The ID is immersed in a 2 T magnetic field produced by the central solenoid magnet in order to measure the transvere momentum of the charged particles. These precision tracking components cover the region |η|<2.5.
Fig. 3.4 Schematic diagram showing a cutaway view of the ATLAS inner detector (ID) and its sub-components during Run 1 [75]. An additional pixel layer known as the IBL was inserted for Run 2.
3.3.1
Pixel Detector and Semiconductor Tracker
During Run 1
The silicon pixel detector offers the highest granularity, and is located closest to the beam pipe. It provides the first spatial measurement of charged particles produced from the IP. It is arranged as three concentric cylindrical layers around the beam pipe in the barrel region |η| < 1, and as disks of three layers perpendicular to the
beam axis in the endcap regions. The pixel layer closest to the beam pipe is critical for B-physics studies, in order to best resolve the secondary vertices from long-lived b-hadrons, and is known as theB-layer. The pixel detector contains around 80 million
3.3 The Inner Detector 36
read-out channels: around 67 million in the cylindrical barrel layers and 13 million in the endcap layers, with each pixel sensor having size R−ϕ×z = 50×400 µm2 with
resolution R−ϕ×z = 10×115 µm2 [75]. The pixel detector covers the radial region
50.5< r <150.0 mm.
The SCT consists of 4088 silicon strip modules. These are arranged as four concentric cylindrical layers around the beam pipe in the barrel region. Each of the two endcaps contain nine disk layers perpendicular to the beam direction, which can be seen in Figure 3.4. The SCT provides four spatial measurements for traversing charged particles, from eight strip measurements. In the barrel region, each module consists of two 6.4 cm silicon strip sensors daisy-chained together with a pitch of 80 µm, on each side of the module [75]. The strips on each side of the module have
a small 40 mrad angle between them, to perform a stereo angle measurement. The typical resolution of each module in the SCT is (R−ϕ , z) = (17, 580)µm in the
barrel, and (R−ϕ , R) = (17, 580)µm in the endcaps, covering the radial region
299< r <514 mm.
Run 2 Upgrade
The high-luminosity, high-radiation environment close to the IP has the effect of degrading the pixel detector, particularly the B-layer. The lifetime in integrated
luminosity of the present B-layer due to these radiation effects is estimated to be
around 300 fb−1 [78], with the onset of tracking efficiency degradation at an even lower
integrated luminosity. For Run 2, an additional pixel layer was inserted closest to the beam pipe, known as the Insertable B-Layer (IBL). A comprehensive technical report of the IBL can be found in [78]. The IBL is expected to maintain robust tracking until the replacement of the inner detector in 2025. The IBL also provides improved precision for vertexing and b-tagging.
3.3.2
Transition Radiation Tracker
The TRT is the outermost component of the ID, and consists of roughly 298,000
proportional drift tubes. The tubes have a 4 mm diameter, with a wall made from polyimide, offering good electrical and mechnical properties with minimal thickness at around 35 µm. The walls are kept at a voltage of −1.5 kV [75]. The tubes are filled