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

event is also discarded. Events with noise bursts in the LAr calorimeter are removed. Events with exactly
three leptons, two ID leptons and one exclusive TL, are selected. The three leptons have to be matched
to the same good primary vertex, which suppresses events from pile-up and secondary interactions. The
TL is required to have pT > 25 GeV, whereas the ID leptons are required to have pT > 20 GeV. At least
two ID jets and E miss
T
> 20 Gev are required. At least one of the ID jets in the event must be b-tagged.
The preselection is completed by requiring at least two leptons of the same flavor and opposite charge, to
account for the Z-boson from the FCNC decay of one of the top quarks. For this requirement, the TL (of
unknown flavor) it is assigned the flavor of the lepton with which it is paired. In 2.05 fb −1 there are a total
of 28 events passing the preselection criteria and an expected background of 26.5 ± 4.9. The details of the
background evaluation are given in Chapter 6. Distributions of relevant variables obtained after these cuts
are shown in Figure 5.9.
Event Reconstruction
To complete the event selection the mass of the two top quarks can be reconstructed from the corresponding
decay particles, taking advantage of the energy-momentum conservation. Background events are expected to
fit less well with the t¯t hypothesis, this provides an opportunity for furher signal and background separation.
Given that the neutrino from the SM single lepton decay of the top quark (t → Wb → bℓν) goes undetected,
its four-momentum needs to be evaluated. This can be done by assuming the E miss
T
to be the neutrino
transverse momentum, p ν T . The longitudinal component of the neutrino momentum (pν z) is then determined
together with the choice for the jet and lepton combination by minimizing the following expression:
χ 2 =
mreco 2 − mt
jaℓaℓb
σ 2 t
where mreco , mreco
jaℓaℓb jbℓcν , mreco
ℓcν
+
mreco 2 jbℓcν − mt
σ 2 t
+
m reco
ℓcν
2 − mW
σ 2 W
+
reco 2
m − mZ ℓaℓb
σ2 , (5.2)
Z
and mreco,
are the reconstructed masses of the top quark candidate decaying
ℓaℓb
through the FCNC decay, the top quark candidate with SM decay, the W boson candidate from the top
quark with SM decay and of the Z boson candidate from the top quark FCNC decay, respectively. There
the masses are mt = 172.5 GeV, mW = 80.4 GeV, and mZ = 91.2 GeV, and the widths are σt = 14 GeV,
σW = 10 GeV and σZ = 3 GeV. For each ID jet and lepton (the two ID leptons and the unique TL) com-
bination, in which the Z-boson candidate is built from two leptons of the same flavor and opposite charges,
the χ 2 minimization gives the value for p ν z. From all combinations, the one with the minimum χ 2 is chosen
along with the corresponding p ν z value. Only the two leading ID jets are used in the reconstruction of the
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