CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

analysis, is 22%. Figure 5.13(a) shows the generator channel on these 2ID+TL events and Figure 5.13(b)
shows the distribution in η of the TLs accepted by the analysis. The gain in acceptance comes partly from
recovery of efficiency losses in the e and µ selection, partly from acceptance of hadronic τ decays, but pri-
marily from electrons and muons in the transition regions and gaps in detector coverage. In Figure 5.13(b),
the gap in muon coverage around η ∼ 0, and between the A and C sides of the spectrometer is clearly seen,
as well as other more subtle features of the detector.
decay fraction of events accepted [%]
t¯t → ℓνℓℓ 2.60 ± 0.05 +0.32
−0.37
t¯t → ℓνττ,τνττ 0.04 ± 0.01 +0.02
−0.02
t¯t → τνℓℓ 0.94 ± 0.04 +0.17
−0.18
+0.006
signal efficiency 0.045 ± 0.001 −0.007 %
Table 5.6: ℓ = e,µ. Fraction of events accepted per signal MC sample. The listed uncertainties are
statistical and systematic, respectively. The signal efficiency quoted at the bottom has been convoluted with
the corresponding Z and W BRs.
trk­leptons
20
18
16
14
12
10
8
6
4
2
0
ATLAS Internal
2ID + TL
after b­tagging
after χ2
mass cuts
Z( ττ)W(
τν)
Z( ττ)W(
µ ν)
Z( ττ)W(eν)
Z( µ µ )W( τν)
Z(ee)W( τν)
Z( µ µ )W( µ ν)
Z( µ µ )W(eν)
Z(ee)W( µ ν)
Z(ee)W(eν)
(a)
Generator channel
trk­leptons
7
6
5
4
3
2
1
0
ATLAS Internal
2ID + TL
after b­tagging
after χ2
mass cuts
­2 ­1.5 ­1 ­0.5 0 0.5 1 1.5 2
(b)
η [rad]
Figure 5.13: (a) Generator channel of the 2ID+TL events selected before and after the χ 2 mass cuts and
(b) η distribution of the TLs of these events. These TLs are exclusive of any electrons or muons. The units
in the y-axis are arbitrary, but the corresponding BRs for the W- and the Z-boson decays have been taken
into account.
To examine the effect of pile-up on the acceptance, the acceptance is measured as a function of the mean
number of interactions per bunch crossing (〈µ〉). This is shown in Figure 5.14. There the black points
are before, and the magenta stars after, pileup reweighting. While there is some scatter, it looks rather
statistical in nature, and there is no dramatic dependence on 〈µ〉. The points after pileup re-weighting are
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