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

Jets in this analysis are reconstructed with the anti-kt algorithm with a radius parameter R = 0.4 [99],
starting from energy clusters in the calorimeter reconstructed using the scale established for electromagnetic
objects. Jets are then calibrated with MC simulation-based pT- and η-dependent correction factors to
restore them to the full hadronic energy scale [100]. Jet quality criteria are applied to reject jets that could
not be associated to in-time real energy deposits in the calorimeters. These faulty jets could arise from
various sources ranging from temporary hardware issues in the calorimeter, the LHC beam conditions and
the showers induced by cosmic-ray muons. Jets are removed if they are within ∆R = 0.2 of a well-identified
electron candidate, or within ∆R = 0.4 of a TL candidate. Finally, jets are required to have pT > 25 GeV
and |η| < 2.5 (in the following referred to as ‘ID jets’).
Momentum Resolution
For the case of jets in the MC, a correction on the energy resolution is also applied to match that of the
data. This smearing is done before the jet selection.
5.3.5 Missing Transverse Energy
A good measurement of E miss
T is an important requirement for the successful identification and reconstruction
of t¯t events. Mismeasured E miss
T
no real E miss
T
is present. The E miss
T
can significantly enhance backgrounds, like multijets or Drell-Yan, where
for this analysis is formed from the negative vector sum of transverse
momenta of all jets with pT > 20 GeV and |η| < 2.5. The contribution from cells associated with electron
candidates is replaced by the candidates’ calibrated transverse energy. The contribution from all ID muons
and other calorimeter clusters, including those not belonging to a reconstructed object, is also included. To
suppress backgrounds from Drell-Yan decaying into a pair of muons, where one muon is a TL that is not
accounted for in the E miss
T
reconstruction, the E miss
T
ID muon, with the pT of the TL if the ∆φ between the E miss
T
is corrected in events with a TL and an opposite-sign
and the TL direction is less than 0.15 and
there is no ID lepton within ∆R < 0.05 of the TL (in which case the correction already has been taken care
of by the E miss
T
algorithm). The correction is as follows:
Figure 5.7 shows the E miss
T
E miss
x
E miss
y
→ E miss
x − p TL
x ,
→ E miss
y
− p TL
y .
(5.1)
distribution for Drell-Yan events selected in data, where the effect of this
correction can be fully appreciated. In the majority of cases of the 2ID+TL events selected in the data, the
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