Inner Detector

The Inner Detector (ID) is contained in a cylinder of radius 1.15 m and length of 7 m, embedded in a 2 T solenoid magnetic field.

The required momentum and vertex resolutions need high-precision measurements to be performed using fine-granularity detectors, given the very large track density of LHC.

These detectors also have to be radiation-hard in order to work for at least ten years. The highest granularity is obtained using Semi-Conductor Tracker (SCT) and pixel detectors.

The SCT Barrel region comprises four cylindrical microstrip layers of silicon microstrips modules and nine disks in the End-Cap, while the pixel detectors are made up of four cylindrical layers of silicon pixels in the Barrel region and of three disks in the End-Cap.

The total number of precision layers must be limited because of the quantity of additional material they introduce, which can perturb the measurements of the properties (energy and momentum) of the particles produced in the collisions. Another reason is related to the cost of such layers. In association with this high precision instrumentation, the Transition Radiation Tracker (TRT) provides a larger number of tracking points, requiring much less material per point and a lower cost.

3.3. ATLAS Experiment Overview The ID combines discrete high-resolution semiconductor pixel and strip detectors in the inner part, and continuous straw-tube tracking detector with transition radiation capability in the outer part. This layout can be seen in Fig. 3.4.

In summary, in the ID the following technologies are present:

• The pixel detectors determine the primary vertex of the collisions and allow mea-suring secondary vertices coming from the long lived particles such as Bhadrons andτleptons. The system contains a total of 140 million detector elements, giving a very high-granularity and precision resolution.

• The SCT consists of layers of silicon microstrip detectors designed to contribute to the measurements of momentum, impact parameter and vertex position, providing also a good pattern recognition using a high granularity.

• The TRT is based on the use of straw tube detectors, which can operate at very high rates thanks to their thin size and isolation of the sensitive wires within individual volumes filled with gas. The electron identification is enhanced employing Xenon gas to detect transition radiation photons created in a radiator between the straws.

TRT provides a good discrimination between electron and hadron signals.

The ID layout provides full tracking coverage over|η| ≤2.5 and provide an experimental resolution of 10 x 115µm for the particle position. The ID reaches a designed resolution of the track momentum of:

σpT

pT

= 0.05%× pT(GeV)⊕0.1%. (3.5)

A fourth pixel layer has been installed in the ID in 2014 to recover the loss of sensitivity of the Pixel Detector due to radiation damage. This Insertable B-Layer (IBL) has been installed between the beam pipe and the Pixel Detector. The internal radius of IBL is 31 mm and the outer one is 38.2 mm, the sensors are present at a radius of 33.4 mm and face the beam pipe on the range of|η| < 2.5. With its 50 x 250µm pixels, the IBL adds additional 12 million pixels to the overall Pixel Detector. The physics performance of the ATLAS detector highly depends on the capabilities of the IBL, which provides an improved vertexing and a betterb-tagging [36].

3. CERN, LHC and the ATLAS Experiment

Figure 3.4.: A view of the Inner Detector layers.

3.3.3. Calorimeters

The ATLAS calorimetry system is designed to serve in a very harsh environment of proton-proton collisions, in particular it has to be efficient at the high luminosity of LHC.

The overall structure of the ATLAS calorimeters is shown in Fig. 3.5.

The barrel Electromagnetic (EM) calorimeter is a highly granular Lead/Liquid-Argon (LAr) sampling calorimeter, see Fig. 3.6. It has a good energy and position resolution and covers the pseudorapidity region of |η| < 3.2. The EM calorimeter is housed in a barrel cryostat, it surrounds the ID, in front of the solenoid which generates the 2 T mag-netic field. The calorimeter is also very important for particle identification and hadronic-electromagnetic separation (γ/π⁰,e/πseparation, etc.). It also provides a precise position measurements inηthrough high granularity. The design of the EM calorimeter is an ar-rangement of absorber layers and active layers in a characteristic accordion geometry, see Fig. 3.6b.

The ATLAS hadronic calorimeters cover the range |η| < 4.9 using different techniques suited for the varying requirements and radiation environment over the largeη-range. The bulk of hadronic calorimetry is given by the Iron-scintillator tile calorimeter, which is

3.3. ATLAS Experiment Overview

Figure 3.5.: A view of the ATLAS calorimeters.

separated in a large barrel and two smaller extended barrel cylinders, one for each side of the barrel, as shown in Fig. 3.6c. The Hadronic End-Cap (HEC) calorimeter and the high density Forward CALorimeter (FCAL) share the LAr technology and are integrated in the same cryostat, which houses the EM end-cap, see Fig. 3.7.

The coverage of the hadronic calorimeter guarantees a good missing transverse momen-tum measurement, which is very important for many physics signatures and also for the detection of SUSY particles. The energy resolution of the calorimetry system is sum-marised in Table 3.2.

Detector Component Energy resolution EM Calorimetry 10%/√

E⊕0.7%

Hadronic Calorimetry

Barrel & End-Cap 50%/√

E⊕3%

Forward 100%/√

E⊕3.1%

Table 3.2.: Nominal detector performance goals for the ATLAS calorimetry system.