Table 2: Performance goals of the ATLAS detector. The units for energy and momentunm are in GeV [35].
Detector component Required resolution ηrange
measurement Trigger
Tracking σpT/pT =0.05%, pT ⊕1% ±2.5 -
EM calorimetry σE/E=10%,
√
E⊕ 0.7% ±3.2 ±2.5
Hadronic calorimetry (barrel and end cap) σE/E=50%, √
E⊕3.0% ±3.2 ±3.2
Hadronic calorimetry (forward) σE/E=100%, √
E⊕10.0% 3.1<|η|4.9 3.2<|η|4.9 Muon spectrometer σpT/pT =10.% at pT =1 TeV ±2.7 ±2.4
3.5 Inner Detector
The Inner Detector is located around the beam pipe at the collision point, and covers a range of 5 < r < 120 cm and |η| < 2.5. It consists of three sub-detectors, two silicon based detectors; the Pixel Detector and the SiliCon Tracker (SCT), and a straw tube gaseous detector; the Transition Radi- ation Tracker (TRT), all of which are surrounded by the inner solenoid, a 2T magnet positioned on the inner side of the electro-magnetic calorimeter. These are shown in fig. 7.
Figure 7: The cut away view of the inner detector with the sub systems labelled [28].
The three sub-detectors are used to determine the location of the primary vertex and any secondary vertices, to aid in particle identification and for charged particles to measure both the momentum and
The ATLAS Detector 3.5 Inner Detector
withstand the high radiation environment that it will be subjected to during data taking, and unless stated otherwise all components are built to survive at least ten years of operation at the LHC.
To ensure good track parameter resolution the location of the sensory elements must be known to within a few micrometers. This is mostly achieved by an alignment procedure using tracks. The SCT also has a built in interferometer based alignment monitoring system [36] that under pins these regular track based alignment procedures.
The amount of material within the ID is kept to a minimum as any materials traversed by an outgoing particle can cause Coulomb scattering, bremsstrahlung, photon conversions or secondaries from nuclear reactions, all of which can effect the accuracy of the track measurement. The amount of material in each sub-detector is shown as a function of ofηin fig. 8.
| η | 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ) 0 Radiation length (X 0 0.5 1 1.5 2 2.5 | η | 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ) 0 Radiation length (X 0 0.5 1 1.5 2 2.5 Services TRT SCT Pixel Beam-pipe
Figure 8: Cumulative amount of material in terms of radiation length for the Inner Detector as a function of
|η|[37].
Another source of error on the track measurements is the exact value of the magnetic field from the solenoid surrounding the ID. Prior to the installation of the ID, after only the barrel and endcap calorimeters were in place, a mobile array of Hall probes were used to map out the magnetic field in the volume to be occupied by the ID. To monitor any changes in this magnetic field during running four Nuclear Magnetic Resonance (NMR) probes are used, located near to z=0 cm.
The inner detector does not contribute to the L1 trigger decision, and therefore all of digitized data from a single event is simply stored in a buffer and only passed to the offdetector electronics if the L1 trigger accepts the event.
The ATLAS Detector 3.5 Inner Detector
3.5.1 Pixel detector
Located closest to the beam pipe is the pixel detector. The pixel detector consists of 1744 identical pixel sensors, spread out across three barrel layers and two lots of three end cap disks. Each pixel sensor has 47232 pixels, each of size 50×400 µm, and is bump bonded to an element of the front end readout integrated circuit [34]. The three barrel layers are concentric cylinders around the beam axis located between 50.5<R<122.5 mm. The proximity of the barrel layers to the beam means that the innermost layer will have to be replaced after about three years of running due to radiation damage. The end cap disks are aligned perpendicular to the beam axis, and are located at both sides A and C of the detector. These are also 250 µm thick. The pixel layers are segmented in R−φand z, and typically three pixel layers are crossed by each track. The intrinsic measurement accuracies for each of the layers and disks are 10µm in the R−φplane and 155µm along the z axis, which is sufficient for high precision tracking measurements.
3.5.2 Silicon Tracker (SCT)
Additional tracking measurements are provided by the SCT, which is located further out from the beam than the pixel detector, and again consists of a barrel region and two end caps. Located at 255 < R < 549 mm is the barrel region, which consists of four cylindrical layers. There are 2112 barrel SCT modules shared out across the four layers. Each module consists of four silicon sensors, two of each on the top and bottom, all with 80µm pitch micro-strip sensors.The front and back sensors are aligned with a stereo angle of 40 mrad and are connected to binary signal readout chips. The shallow stereo angle reduces the number of ambiguities for a particle passing through a module, and also sim- plifies the geometrical layout of the module. The modules are orientated such that the bottom sensor is aligned with the beam line. The precision of each of the barrel SCT modules in the R−φco-ordinate is 17µm and 580µm for the z co-ordinate.
In order to maximise the η coverage there are also nine disk layers in each of the two end caps arranged perpendicular to the beam axis. This ensures that there are at least four precision space-point measurements for each track within the fiducial detector coverage. The layout of the modules in the end caps is such that the accuracy of each of the end cap SCT modules in the R−φco-ordinate is 17µm and 580µm for R.
In order to maintain an acceptably low level of noise during data taking and reduce increases in the required bias voltage the SCT is kept at a temperature around 0◦C.