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

Chapter 2: The Accelerator and the Experiment 51
longitudinal segments. The first segment is optimized for electromagnetic measure-
ments, using copper as the absorber. The second and third segments make hadronic
measurements and use tungsten absorbers. Each segment contains a metal matrix
consisting of holes through which electrode structures have been inserted (Figure 2.7).
An electrode is made of a coaxial copper rod and tube arrangement, the gap between
the rod and the tube being filled with liquid argon.
Since the FCal is very close to the beampipe, it can potentially suffer from high
occupancy. The rod-tube gaps are therefore made very small to minimize ion buildup.
The total depth of the FCal is about 10 λ.
2.2.4 The Muon Spectrometer
The muon spectrometer (MS) is the outermost subdetector of ATLAS, and de-
termines its overall length and diameter. The function of the muon spectrometer is
to measure the position and momenta of particles that exit the calorimetry. Most of
these particles will be muons and, in what follows, the term ‘muons’ will refer to all
particles traversing the muon system.
As discussed in Section 2.2.1, the muon system contains superconducting air-core
toroidal magnets. Muon tracks bend in this magnetic field, so that their momenta
can be reconstructed. The momentum measurement can be improved by matching
tracks seen in the muon system with those reconstructed in the inner detector (Chap-
ter 4). However, it is also possible to reconstruct tracks in the standalone mode using
measurements from the muon spectrometer alone. Standalone track reconstruction is
feasible over a wide range in transverse momentum from ≈ 4 GeV to ≈ 3 TeV.