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

Chapter 4: Data Collection and Event Reconstruction 140
The MDT tube hit efficiency in ATLAS has been estimated using cosmic muons.
Figure 4.12 shows the tube hit efficiency in one MDT chamber (BML2A03) as a
function of the distance of the particle trajectory from the anode (the drift radius) [26].
Efficiency curves are shown for three different cuts on the hit residual R, requiring
R < nσ, for n = 3, 5 and 10, where σ is a convolution of the tube spatial resolution
and the track extrapolation error given by the extrapolator, and R is defined as:
R = di − ri
(4.8)
where di is the distance of the reconstructed track from the anode and ri is the
measured drift radius. As can be seen, even in the most conservative case (red curve),
the hit efficiency is ≈94% or better over most of the drift radius.
Figure 4.13 shows the distribution of tube efficiencies for the tubes in all chambers
in the middle MDT station. Distributions are shown separately for the four longi-
tudinal segments of the muon spectrometer: barrel A and C and endcap A and C.
In each segment, the overall tube efficiency is 93-94%, as expected from studies with
test beam and drift tube simulation [110].
The MDT tube hit efficiency has a systematic uncertainty, due mainly to the
presence of dead tubes. To estimate the number of dead tubes, we used monitoring
histograms from events accepted by the Level-1 trigger produced by the Gnam appli-
cation [41]. We found 590 tubes to be consistently dead, while a further 48 tubes
were dead in about half the runs used in this analysis. Comparing these results with
an offline database containing a list of dead tubes, we found 339 additional tubes to
be dead. Hence, 939 tubes were dead for at least part of the data we use. This is ≈
0.3% of the total number of MDT tubes, and consequently we take this number as a