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

]
­1
Total Integrated Luminosity [fb
3.5
3
2.5
2
1.5
1
0.5
ATLAS Online Luminosity
LHC Delivered
ATLAS Recorded
­1
Total Delivered: 2.68 fb ­1
Total Recorded: 2.55 fb
s = 7 TeV
0
02/03 31/03 29/04 28/05 26/06 25/07 24/08
Day in 2011
Figure 5.8: Cumulative luminosity versus day delivered to (green), and recorded by ATLAS (yellow) during
stable beams and for pp collisions at 7 TeV center-of-mass energy in 2011.
5.6 t¯t Signal
As mentioned above, in order to reduce contribution from multijet background, only leptonic decays of the
W- and the Z-boson are considered signal. Therefore, for the event selection, three high pT leptons, at
least two jets and relatively high E miss
T
are required. The event selection is finalized with the reconstruction
of the two top quarks, and corresponding constrainst on their masses. There are two base-line analyses
for this search. The first one, ‘2ID +TL’ selects two isolated, fully reconstructed ID leptons, and one TL,
whereas the second one, ‘3ID’, selects three ID leptons. Given that the selected TL in the 2ID+TL analysis
is required to be exclusive of any electron or muon selected by the 3ID analysis, these two channels are
orthogonal. The 2ID+TL channel increases the acceptance with respect to the 3ID channel alone by 22%.
This thesis fully describes the 2ID+TL analysis, however, a brief description of the 3ID analysis is presented
in Section 5.6.2 for completeness, given that the final result arises from a combination of both channels.
5.6.1 2ID + TL Analysis
Event Preselection
At the preselection level, events are required to pass one of the single-lepton triggers, as described in Sec-
tion 5.2, with the corresponding matching lepton candidate. To reject events from non-collision background,
events are also required to have at least one good primary vertex. Events are discarded if any jet with
pT > 20 GeV fails quality cuts designed to reject jets arising from calorimeter noise or activity inconsistent
with the bunch-crossing time [100,102]. If an electron candidate and a muon candidate share a track, the
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