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

Chapter 4: Data Collection and Event Reconstruction 139
section, we look at MDT tube efficiency and muon track reconstruction efficiency.
Trigger efficiencies will be addressed in Chapter 6 20 .
Drift tube efficiency
As discussed in Chapter 2, a monitored drift tube registers a hit from a passing
muon by detecting the leading edge of the electron avalanche. There are two ways in
which the tube can fail to register a muon hit.
• The ionization from a charged particle has statistical fluctuations. For a given
particle passage, the ionization can be small enough that the charge collected at
the anode does not exceed the discriminator threshold. Therefore, the passage
of the particle does not register. This is particularly true of muons passing close
to the tube wall such that the path length in the gas is small.
• Following the arrival of a signal at the anode, a drift tube goes into a dead
time of the order of 800 ns during which interval it does not record further hits.
If a δ-electron traverses the tube almost concurrently with the muon, and the
signal from the δ-electron arrives at the anode before the signal from the muon,
the tube registers the δ-electron and fails to register the muon. Indeed, the
inefficiency of single drift tubes is expected to be mostly due to δ-electrons.
The hit efficiency of a drift tube is defined as the ratio of the number of hits found
in the tube in a sample of events to the number of hits expected. A hit is expected
in a tube if a reconstructed track passes through the tube.
20 Since we use only a hardware (Level-1) trigger in our analysis, we will not discuss efficiencies of
the Level-2 and Level-3 triggers.