Calorimetry

which is designed for a lifetime of ten years, the innermost pixel layer (B-layer) has to be replaced after several years of operation at the nominal luminosity [50,64].

A comprehensive description of the ATLAS ID can be found in Ref. [50], in the TDRs of the ID [65, 66] and pixel detector [67], as well as in Refs. [64,68,69]. A brief summary of the ID sub-detectors is given in the following.

The pixel detectormeasures charged particles using silicon sensors (pixels), which have a min-imum size of 50 ×400µm². In the barrel region, the pixels are arranged on three concentric cylinders around the beam axis (45.5 mm< R <242 mm), while in the end-cap regions they are located on five disks (on each side) perpendicular to the beam axis (88.8 mm< R < 149.6 mm).

Typically, three pixel layers are crossed by each track. The intrinsic accuracy is:10µm,115µmin (R−φ),zcoordinates, respectively, in the barrel; and10µm,115µmin(R−φ),Rcoordinates, respectively, in the two disks. The pixel detector has a total number of approximately80.4million readout channels. The B-layer defines essentially the secondary vertex measurement accuracy since it permits measurements at the smallest radius of∼5cm.

The SCT is a silicon microstrip detector. Its geometry is similar to the pixel detector: Four cylindrical layers are located at 299 mm < R < 514 mm and nine disks (on each side) at 275 mm < R < 560 mm. Each track crosses eight SCT strip layers (four space points). In the barrel region, the SCT uses small-angle (40mrad) stereo strips to measure both coordinates, with one set of strips in each layer parallel to the beam axis. They consist of 6.4m long daisy-chained sensors with a strip pitch of80µm. In the end-cap region, the detectors have a set of strips running radially and a set of stereo strips at an angle of40mrad. The mean pitch of the strips is also approximately80µm. The intrinsic accuracy is: 17µm,580µmin(R−φ),zcoordinates, respectively, in the barrel; and17µm,580µmin(R−φ),Rcoordinates, respectively, in the disks.

The total number of readout channels in the SCT is approximately6.3 million.

TheTRTconsists of straw tubes,4mm in diameter, that cover a range up to|η|<2. On average, 36 hits are provided by the TRT. It measures precisely onlyR−φcoordinates with an intrinsic accuracy of130µmper straw. In the barrel region, the144cm long straws are parallel to the beam axis. Their wires are divided into two halves at approximatelyη = 0. In the end-cap region, the 37cm long straws are arranged radially in wheels. The total number of TRT readout channels is approximately351thousand. The TRT’s capability to detect transition-radiation photons enhances the overall ATLAS electron identification performance.

The three independent sub-detectors of the ID are complementary: the combination of precision trackers at small radii together with the TRT at larger radius gives robust pattern recognition and high precision in bothR−φandzcoordinates. The TRT straw hits contribute significantly to the momentum measurement, since the lower precision per point is compensated by the larger number and longer measured track length.

3.2.3 Calorimetry

The ATLAS calorimeter system, shown in Fig. 3.5, covers a range of |η| < 4.9using different techniques suited to the widely varying requirements of the physics processes of interest and of

Figure 3.5: Cut-away view of the ATLAS calorimeter system [50].

the radiation environment over this large η-range. It accommodates an electromagnetic (EM) calorimeter, a hadronic calorimeter, and forward calorimeters (FCal).

A full description can be found in Ref. [50] and in the ATLAS calorimeter TDRs [70,71,72]. In the following, the sub-detectors are briefly described.

TheEM calorimeteris composed of a barrel covering the region|η|< 1.475 and two end-caps (EMEC) covering the region1.375 < |η| < 3.2. It is a liquid-argon (LAr) sampling detector with accordion-shaped kapton electrodes and lead absorber plates. The novel accordion geometry provides complete φsymmetry without azimuthal cracks. The barrel and two EMECs are each housed in their own cryostat. The barrel is further divided into two identical half-barrels, separated by a small gap (4mm) atz = 0. Each EMEC is mechanically divided into two coaxial wheels.

The total thickness of the EM calorimeter in terms of radiation lengths (X0) is>22in the barrel and>24in the end-caps.

The segmentation of the EM calorimeter is as follows: For precision measurements within|η|<

2.5, matched to the ATLAS IDη-coverage, the EM calorimeter is segmented into three longitudi-nal sections with varying granularities depending onη. The middle section, for instance, consists of square towers of ∆η ×∆φ = 0.025×0.025 for|η| < 2.5, see Fig. 3.6. The EMEC inner wheel (|η|>2.5) is segmented into two longitudinal sections and has a coarser lateral granularity.

A presampler detector (active LAr layer) is used to correct for energy losses due to up-stream material.² It covers the region|η|<1.8.

In total there are more than170thousand readout channels in the EM calorimeter.

2There is approximately2X0(2.5X0) material in front of the presampler (first layer) atη= 0[50].

3.2.3 Calorimetry 25

∆ϕ = 0.0245

∆η = 0.025 37.5mm/8 = 4.69 mm

∆η = 0.0031

∆ϕ=0.0245 36.8mmx4x4

=147.3mm

Trigger Tower

Trigger Tower

∆ϕ = 0.0982

∆η = 0.1

16X₀

4.3X0

2X0

1500 mm

470 mm

η ϕ

η = 0

Strip cells in Layer 1

Square cells in Layer 2 1.7X0

Cells in Layer 3

∆ϕ×∆η = 0.0245×0.05

Figure 3.6: Sketch of an EM calorimeter barrel module. The granularity in eta and phi of the cells of each of the three layers and of the trigger towers is indicated.

Thehadronic calorimeterconsists of a barrel covering the region|η|<1.0, two extended barrels covering the region 0.8 < |η| < 1.7, and two hadronic end-caps (HEC) covering the region 1.5 < |η| < 3.2. The barrel and extended barrels are sampling tile detectors, using steel as the absorber and scintillating tiles as the active material. They are hence named Tile barrel and Tile extended barrels. The HEC is a sampling LAr detector.

The Tile barrel and extended barrels are placed directly outside the EM calorimeter envelope. All Tile calorimeters are divided azimuthally into 64 modules and longitudinally into three layers, with approximately1.5,4.1, and1.8interaction lengths (λ) thick for the barrel and1.5,2.6, and 3.3λfor the extended barrels. The scintillating tile is read out by wavelength shifting fibres into two separate photomultiplier tubes. The granularity is∆η×∆φ= 0.1×0.1(0.2×0.1) for the barrel and extended barrels in the first two layers (last layer).

The HEC calorimeter consists of two independent wheels per end-cap, located directly behind the EMEC and sharing the same LAr cryostat. Each wheel is built from32 identical wedge-shaped modules, and is divided into two longitudinal segments. The granularity is∆η×∆φ= 0.1×0.1 (0.2×0.2) in the region1.5<|η|<2.5(2.5<|η|<3.2).

The FCal is a LAr copper or tungsten detector and is integrated into the end-cap cryostats. It is approximately10λdeep, and consists of three modules in each end-cap: the first , made of copper, is optimised for EM measurements, while the other two, made of tungsten, measure predominantly the energy of hadronic interactions.

The ATLAS calorimeters, with 22 −24X₀ and about 10λfor the EM and hadronic detectors respectively, provide good containment for EM and hadronic showers³, as well as limited punch-through into the muon system. Including the1.3λfrom the outer support, the total thickness is approximately 11λ atη = 0. Together with the large η-coverage, this thickness ensures good missing energy measurements, which is important in particular for Supersymmetry searches.

3Important to provide good energy resolution, also for high-energy jets.

Figure 3.7: Cut-away view of the ATLAS muon spectrometer [50].