Measurement of the Z boson cross-section in - Harvard University ...

Chapter 2: The Accelerator and the Experiment 45
where the first term is due to multiple scattering, and the second term corresponds to
the intrinsic resolution of the detector components. Therefore, for a 100 GeV track,
the pT resolution is ≈ 5%, while it degrades to ≈ 35% for a 1 TeV track 2 .
2.2.3 The Calorimetry
The main purposes of the ATLAS calorimetry are to measure the energies of elec-
trons, photons and jets with as high a precision as possible, and to provide missing
transverse energy (Emiss T ) measurements. Jet reconstruction in complex multijet en-
vironments is a crucial requirement. These goals are important for most physics pro-
grams, requiring very good hermeticity of the calorimeters. In addition, the calorime-
try must ensure that showers are well-contained within its volume, and that leakage
of hadrons (hadronic punch-throughs) into the muon system is minimal.
The design of the sampling calorimeters in ATLAS is based on repeated units
consisting of absorber layers, in which particles produce showers, followed by sensitive
volumes that estimate the energy of shower particles. As usual, the calorimetry has an
electromagnetic (EM) component and a hadronic component, with a pseudorapidity
coverage |η| < 4.9. Figure 2.5 shows a cutaway view of the ATLAS calorimetry. It
has four subsystems, the main features of which are described below.
The Liquid Argon (LAr) Electromagnetic Calorimeter
The electromagnetic calorimeter measures the energy and position of electrons
and photons. It has a barrel section, covering |η| < 1.475, and two endcap sections,
2 Using cosmic tracks, the momentum resolution of the ID was found to be ≈ 5% for 100 GeV
tracks. See Figure 23 in [97].