Calorimeter System

The purpose of the ATLAS calorimeter system is to measure the energy of charged and neutral particles, such as photons, electrons, τ-leptons, as well as jets originating from quarks and gluons over a range from a few GeV up to several TeV. It is also used to measure an energy imbalance of an event, which corresponds to missing transverse energy and hence particles that escape the detector undetected¹¹. The calorimeter is segmented into cells transversal and along the particles’ direction of flight through the detector and its different components can be seen in figure?? (left). In the central region of the detector (|η| <3.2) the electromagnetic calorimeter has a fine granularity and provides a high resolution for energy measurements of photons and electrons, while the hadronic calorimeter outside the electromagnetic calorimeter and in the forward region contains coarser segments, still providing enough information for precision measurements of jets and missing transverse energy. The forward hadronic calorimeter, designed to compensate for radiation damage in the environment close

10compared to 3 in the pixel detector and 4 to 9 in the SCT detector

11Neutrinos will cause missing transverse energy, but also many particles in models for physics beyond the Standard Model, such as Supersymmetry.

to the beam, covers the detector space up to |η| <4.9. The latter ensures that all particles visible to the detector will be detected and therefore makes the measurements of missing transverse energy possible. An important requirement for a good calorimeter is that all electromagnetic and hadronic

2008 JINST 3 S08003

Figure 1.3: Cut-away view of the ATLAS calorimeter system.

Calorimeters must provide good containment for electromagnetic and hadronic showers, and must also limit punch-through into the muon system. Hence, calorimeter depth is an important design consideration. The total thickness of the EM calorimeter is>22 radiation lengths (X0) in the barrel and>24 X0in the end-caps. The approximate 9.7 interaction lengths (λ) of active calorimeter in the barrel (10λin the end-caps) are adequate to provide good resolution for high-energy jets (see table1.1). The total thickness, including 1.3λfrom the outer support, is 11λ atη=0 and has been shown both by measurements and simulations to be sufficient to reduce punch-through well below the irreducible level of prompt or decay muons. Together with the large η-coverage, this thickness will also ensure a goodET^missmeasurement, which is important for many physics signatures and in particular for SUSY particle searches.

1.3.1 LAr electromagnetic calorimeter

The EM calorimeter is divided into a barrel part (|η|<1.475) and two end-cap components (1.375<|η|<3.2), each housed in their own cryostat. The position of the central solenoid in front of the EM calorimeter demands optimisation of the material in order to achieve the de-sired calorimeter performance. As a consequence, the central solenoid and the LAr calorimeter share a common vacuum vessel, thereby eliminating two vacuum walls. The barrel calorimeter consists of two identical half-barrels, separated by a small gap (4 mm) atz=0. Each end-cap calorimeter is mechanically divided into two coaxial wheels: an outer wheel covering the region 1.375<|η|<2.5, and an inner wheel covering the region 2.5<|η|<3.2. The EM calorimeter is a lead-LAr detector with accordion-shaped kapton electrodes and lead absorber plates over its full coverage. The accordion geometry provides completeφsymmetry without azimuthal cracks. The

– 8 –

Figure 3.10: Distribution of calorimeter material as a function of psuedorapidity [66]. The top shaded region indicates the material in the first layer of the muon system, and the bottom shaded region indicates the material in the inner detector.

energy. The measured signal in the liquid argon calorimeters is translated to deposited energy via the equation [69]:

E^cell= 1 fI/E·F·

!N

sample=1

OFsample(Ssample−P). (3.2)

N is the number of samples taken of the pulse, typically five for collision data, but up to 32 for calibration purposes. Ssampleis the signal height, measured in ADC counts, for each sample. The pedestal noise level,P, is derived from data taken taken during periods when no signal events are expected, The optimal filtering coefficients,OFsample, are derived from the expected shape of the analog pulse [70]. They are used to reduce any inherent jitter in the signal shape due to factors such as cross-talk between channels and underlying noise. It is important to note that since these coefficients attempt to reconstruct a signal over a small number of samplings assuming an ideal pulse shape, it is possible for them to cause a negative value of energy to be reconstructed. The observed ADC counts are converted to a current by the factorF, which is determined during calibration by injecting with a well-known current into the readout cell. Finally,fI/E performs the translation from measured current to energy. Due to the complexities of the structure of the electric

Figure 3.5.: Overview of the calorimeter system of the ATLAS experiment [67] (left) and distribution of the material budget in terms of interaction length λ[67] (right).

showers are stopped within the detector volume and that punch-through into the muon system is negligible. This can be achieved by the thickness and density of the material used in the calorimeter and the supporting structure. For this reason, a particle crossing the electromagnetic calorimeter will experience more than 24 (26) radiation lenghts X0 in the barrel (endcap) region. After passing the complete calorimeter a particle has traversed about 11 interaction lengthsλon average, see figure??

(right) for the η-dependence, which sufficiently suppresses punch-through.

Both calorimeter parts are sampling calorimeters, a mixture of passive absorber material with a high number of protons in the nucleus and active detector material to read out the signal caused by energy deposition.

3.7.1. The Electromagnetic Calorimeter

Over the full range of |η| < 3.2 the electromagnetic (EM) calorimeter uses lead plates as passive and liquid argon as active detector material and it is thus often abbreviated as LAr EM calorimeter.

The active detector material is interspersed with Kapton electrodes arranged in an accordion-shaped geometry to ensure full coverage of the phase space in φ without gaps in the azimuthal direction.

Due to this structure each particle crossing the detector will experience about the same amount of material.

The EM calorimeter is divided into three parts, each in their own separate cryostat system: the barrel part of the calorimeter covers the region up to |η| < 1.475, with a subsequent endcap calorimeter for the range 1.375 < η < 3.2 on each side. In addition, a thin liquid argon layer is assembled as presampler in the region up to |η| < 1.8 to correct for inhomogeneous energy losses in the inner detector and its support structure. The barrel part is separated into two half-shells with a 6 mm gap at z = 0, while the endcap parts consist of an inner and an outer wheel divided up into 1.375< |η| <2.5 and 2.5< |η| <3.2. Furthermore, the barrel region of the detector is segmented into three longitudinal layers with thickness of about 4X0, 16X0 and 2X012, counting outwards. The

12atη= 0

innermost layer is very finely binned (∆η = 0.0031) and allows to precisely match calorimeter information with tracking information as well as to distinguish close-by showers. This feature also allows to measure the direction of photons with high precision, for which the tracking detector cannot provide information. The middle layer is designed to contain the main energy deposition of each electromagnetic shower with a granularity of ∆η = 0.025 and the thin outermost layer is used to correct for leakage into the successive hadronic calorimeter.

The energy resolution of the EM calorimeter is given by

∆EE = p11%

E[GeV]⊕0.4%, (3.6)

where the first term describes the statistical fluctuations in the sampling material and the constant second term systematic uncertainties from an inhomogeneous material distribution.

3.7.2. The Hadronic Calorimeters

The hadronic calorimeters are designed to fully contain energetic jets originating from quarks and gluons, that interact via the strong interaction with the nuclei of the material. While the central part (|η| < 1.7) is equipped with a sampling calorimeter using scintillating tiles and iron plates, the forward region (up to |η| < 4.9) consists of a liquid argon hadronic calorimeter and a dedicated forward calorimeter.

3.7.2.1. The Hadronic Tile Calorimeter

In the central region, the hadronic calorimeter consists of a barrel part (|η| <1.0) and two extended barrel calorimeters¹³, covering the region 0.8< |η| < 1.7 and covers the area with an inner radius of ri = 2.28 m up to a radius of ro = 4.25 m. The active detector material is made of polystyrene scintillator tiles with a thickness of 3 mm, alternating with iron plates as passive absorbers with increasing radii. In the longitudinal direction the detector consists of three layers with thickness in terms of interaction length moving outwards of 1.4λ,4.0λ,1.8λat η= 0. The segmentation in η and φ is ∆η ×∆φ = 0.1×0.1 in the innermost two layers and 0.2×0.1 in the outermost layer.

In the Tile Calorimeter, wavelength shifters are attached to each side of a scintillator tile, which are separately read out using photomultipliers. This provides a very fast signal and can be used in the trigger system.

3.7.2.2. The Hadronic LAr Calorimeters

Located in the same cryostat as the endcaps of the EM calorimeter on each side of the detector are the hadronic endcap calorimeter (HEC), covering the region of 1.5 < |η| < 3.2, and a dedicated forward calorimeter (FCAL) in the very forward region close to the beam pipe (3.1< |η| <4.9). Both use the same technology and active material as the LAr EM calorimeter. Since the EM calorimeter also extends up to |η| <3.2, good coverage in the transition region between the different calorimeter

13one on each side

components is achieved.

The HEC consists of two wheels on each side of the detector, both divided into two segments along the longitudinal direction. Copper plates of 25-50 mm thickness are used as absorber material and alternate with 8.5 mm thin gaps filled with liquid argon and the read-out wires. The FCAL is located inside the HEC, towards the beam-pipe, and designed to operate constantly under high radiation exposure due to its location. It consists of three sections, the innermost one using copper as absorber material and the outermost two using tungsten as passive detector material, both alternating with LAr gaps. The choice of material allows to achieve optimal measurements of both electromagnetic and hadronic showers.