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

Chapter 2: The Accelerator and the Experiment 49
sitates a shower sampling preceding the calorimetry. This is accomplished by having a
presampler in front of the EM calorimeter. The presampler covers the region |η| < 1.8,
and consists of an active layer of liquid argon of thickness 1.1 cm in the barrel and
0.5 cm in the endcap. Signal readout occurs via interleaved cathodes and anodes.
The Hadronic Calorimeter
The hadronic calorimeter measures jet energy and position. It has three sections
along the z coordinate: a barrel section, which covers |η| < 1.0, two extended barrel
sections, covering 0.8 < |η| < 1.7, and two endcap sections that cover 1.5 < |η| < 3.2.
Adjacent sections have some overlap in order to ensure that the material density does
not drop near the ends. In the barrel and extended barrel sections, the absorber
is steel and the active material is plastic scintillator tiles. The endcaps use copper
absorber and liquid argon. In terms of the interaction length 4 λ, the depth of the
hadronic calorimeter is ≈ 10λ in the barrel, between 7 and 14 λ in the extended
barrel [14], and between 12 and 16 λ in the endcap.
In the barrel and extended barrel regions, charged particles in hadronic showers
produce light in the tiles in the ultraviolet wavelengths, which is converted to visible
light by scintillator dies. The light propagates to the edges of the tiles, where it
is absorbed in wavelength shifting (WLS) fibers and converted to longer-wavelength
light. The fibers guide the light to photomultiplier tubes (PMTs) where it is amplified
and measured. Readout cells are built by grouping fibers into the PMTs such that the
cells are approximately projective toward the interaction region. The measurement
4 In a given material, the interaction length of hadrons is the mean free path between successive
nuclear interactions.