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

Chapter 2: The Accelerator and the Experiment 53
Precision measurement technologies
Monitored Drift Tubes (MDTs): The MDTs form the principal component
of the muon spectrometer precision measurement system. They cover the region
|η| < 1.0 in the barrel, and 1.0 < |η| < 2.7 in the endcap, except in the innermost
endcap layer where the coverage is up to |η| =2.0. Each chamber consists of several
layers of drift tubes, usually arranged in two multilayers (Figure 2.9). Each drift
tube has a diameter of 3 cm, with a body made of aluminum and a central (anode)
wire made of gold-plated tungsten. The wire has a thickness of 50 µm and carries
a potential of 3080 volts. The tube contains a gas mixture of Ar/CO2 in the ratio
93/7, with a trace of water added to improve high-voltage stability.
A charged particle passing through the gas creates electron-ion pairs (Figure 2.10).
The electrons accelerate toward the anode under the high voltage, creating further
electron-ion pairs and leading to an avalanche. A measurable signal is thus formed.
The time taken for the leading edge of the signal to arrive at the anode gives an
estimate of the distance from the wire at which the muon passed, known as the drift
radius. By fitting a straight line through the drift circles in a given multilayer, a
segment of the muon track can be reconstructed.
The spatial resolution of a single drift tube is ≈ 80 µm in the precision coordinate
(z in the barrel and r in the endcap). The resolution of a chamber is ≈ 35 µm. In the
barrel region, the MDTs are arranged in three concentric cylinders (stations) around
the beampipe, at radii of ≈ 5 m, 7.5 m and 10 m. In each endcap, they are arranged
in three wheels perpendicular to the beampipe, at distances of |z| ≈7.4 m, 14 m and
21.5 m. The arrangement is such that a muon coming from the interaction region