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

Chapter 2: The Accelerator and the Experiment 58
The spatial resolution of an RPC chamber is 10 mm in both η and φ coordinates,
while the time resolution is 1.5 ns. The signal width is only 5 ns, making accurate
bunch-crossing identification possible. The total number of RPC channels is 359,000.
Note that, because of space constraints due to the ‘feet’ of the detector, barrel toroid
support structures and various services for the muon spectrometer, the η −φ coverage
of the RPC chambers is about 80%.
The RPCs are arranged in three concentric stations around the beam axis. There
is one station each on the bottom and top faces of the middle barrel MDT chambers,
known respectively as the low-pT plane and the pivot plane. The third station is on
the outer barrel MDT chambers (Figure 2.12), known as the high-pT plane. If there
is a hit on the pivot plane, the trigger logic checks for hits in the low-pT plane in a
defined road and within the same 25 ns time window as that of the pivot plane hit. If
a hit is found, the logic issues a low-pT trigger, defined as 4 GeV < pT < 10 GeV. In
order to minimize fake decisions, hits in at least three of the four RPC layers in the
two stations are required. If a low-pT trigger has been issued, the logic looks for at
least one hit in the high-pT plane in a defined road and within the same time window.
If a hit is found, a high-pT trigger is issued, defined as pT > 10 GeV.
Thin Gap Chambers (TGCs): In the endcap region, radiation levels are ≈ 10
times higher than in the barrel, so that finer granularity is needed of the trigger
system than RPCs can provide. In addition, in the endcap the track momentum
corresponding to a given value of pT increases faster than the bending power of the
magnetic field, which means an increased trigger granularity is needed to match the