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

event kinematics, however no b-jet identification is used. The distributions of the reconstructed mass of the
Z- and W-boson and the two top quarks, this is m reco
ℓaℓb
Figure 5.10.
, mreco
ℓcν , mreco
jbℓcν and mreco respectively, are shown in
jaℓaℓb
For the final selection, events are rejected unless the reconstructed masses of the two top quark candi-
dates are within 40 GeV of mt, the reconstructed W-boson candidate mass is within 30 GeV of mW, and
the reconstructed Z-boson candidate mass is within 15 GeV of mZ. Of the 2ID+TL events in the data
that pass the preselection, 71% are rejected by the χ 2 mass cuts. The efficiency of the FCNC MC events to
pass the χ 2 selection is 66%, while background events pass at a 34% rate. After the application of the χ 2
minimization and invariant mass cuts there are a total of 8 events in the data and an expected background
of 8.9 ± 2.3. The signal efficiency in the final selection is 0.045 ± 0.001 +0.006
−0.007 %. Table 5.5 gives a cut flow
of the event selection in data. The signal efficiency determination is explained in what follows, and the
backgrounds in Chapter 6. Figure 5.11 shows distributions of relevant variables at the final selection level.
The mass distribution of the W- and Z-bosons and the two top quarks candidates after all mass cuts are
shown in Figure 5.12.
Acceptance
Preselection
Events with at least 2 ID leptons, passing event cleaning cuts 69883
Trigger 69853
Trigger Matching 69853
Exactly 2 ID leptons and one TL, from same primary vertex 320
Z-boson candidate 320
E miss
T
> 20 GeV 129
# jets ≥ 2 55
at least one b-tagged jet 28
Final Selection
mass cuts after χ 2 minimization 8
Table 5.5: Data cutflow.
The signal acceptance is measured using the TopRex signal MC samples described in Section 4.2. These MC
samples are divided according to the leptonic decays of the W- and Z-bosons in the final state. Table 5.6
shows the fraction of events accepted in each category. The final signal efficiency, accounting for the W- and
Z-bosons decay BRs, is 0.045 ± 0.001 +0.006
−0.007 %. The increase in the acceptance, with respect to the 3ID
66