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

Chapter 2: The Accelerator and the Experiment 47
covering 1.375 < |η| < 3.2. The absorber material is lead, and the active medium is
liquid argon. The lead plates and the copper/kapton readout electrodes are arranged
in an accordion shape (Figure 2.6). In terms of the radiation length 3 X0, the depth
of the EM calorimeter is more than 22 X0 in the barrel, and 24 X0 in the endcap.
The barrel contains about 100,000 readout channels, while the endcaps have about
62,000 channels.
Electrons and photons create showers in the lead plates. The shower particles
ionize the liquid argon, the amount of ionization being proportional to the energy of
the incident particle. The copper/kapton electrodes collect the signal. The advantage
of using liquid argon as the active medium is that it is intrinsically radiation-hard
and has a linear behavior with respect to particle energy. The accordion geometry
provides full symmetry in the φ coordinate without cracks, and allows fast signal
extraction at the ends of the electrodes.
The EM calorimeter has three radial segments or layers. Layer 1 (Figure 2.6) has
a depth of 4.3 X0. Its main purpose is to provide excellent resolution in η in order to
separate π 0 decay showers from prompt photon showers.The readout is done in fine
η strips, the strip size being ∆η × ∆φ =0.003 × 0.1. Layer 2 has a depth of 16 X0
and a cell size of ∆η × ∆φ =0.0245 × 0.0245. This layer measures shower position.
The third layer, of depth ≈ 2X0 and cell size ∆η × ∆φ =0.05 × 0.0245, samples the
highest energy electrons and photons.
The energy resolution of the EM calorimeter can be expressed as:
3 For a given material, the radiation length is defined as the distance after traversing which an
electron has 1/e of its original energy left. It is also 7/9 of the mean free path of pair production
for a photon.