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

Chapter 2: The Accelerator and the Experiment 56
readout granularity.
• It must associate a track with a particular bunch-crossing, which necessitates a
timing resolution better than the 25 ns bunch-crossing interval.
• It should measure the coordinate perpendicular to the precisely measured one
(the second coordinate) with a resolution of 1 cm or better.
• The trigger chambers must perform well in the presence of a diffuse background
of low-energy neutrons and photons in the experimental hall (the cavern back-
ground).
The ATLAS muon system uses resistive plate chambers for triggering in the barrel
region and thin gap chambers in the endcaps. Both technologies satisfy the require-
ments mentioned above.
Resistive Plate Chambers (RPCs): RPCs are used for triggering in the region
|η| < 1.05. An RPC unit is formed by two parallel plates made of a resistive material,
Bakelite in this case, with a 2 mm gas-filled gap between the plates. An electric field
of 4900 V/mm exists between the plates, so that a charged particle traversing the gas
gap creates an avalanche. The signal is picked up via capacitive coupling by metallic
strips mounted on the outer sides of the Bakelite plates. A chamber is formed of two
rectangular detector units, each read out by two planes of metallic strips. The strips
in the two planes are orthogonal to each other, so that the precision (η) coordinate
as well as the φ coordinate can be measured 6 .
6 The measurement of the φ coordinate is necessary for track reconstruction, since it gives an
estimate of where along the length of an MDT tube the track passed.