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

Appendix B
Backgrounds to the 3ID analysis
B.1 Monte Carlo Background
Similarly to what is done for the 2ID+TL analysis, the background coming from diboson (WW and ZZ)
plus jets and t¯t + W/Z are estimated from MC simulation.
B.2 Fake Leptons Background
Background to the 3ID candidate events in which at least one jet is reconstructed as a lepton is separated
in two categories: background events with one fake lepton, and background events with two or three fake
leptons. Background events with one fake lepton are evaluated using a combination of data and MC samples.
The Z+jets background is estimated using a DD normalization to the MC estimate. Z+jets, along with
other sources of one fake lepton background, are evaluated at the final selection level using a loose lepton
reconstruction selection and a multiplicative factor to scale to the final selection. This loose selection is
achieved by relaxing the leptons requirements. Loose muons are defined as in Section 5.3.1, but without
the isolation requirement. Loose electrons are obtained with the selection described in Section 5.3.2, but
relaxing the isolation requirement to < 6 GeV. Events with two fake leptons (such as W+jets and single lep-
ton t¯t events) or three fake leptons (such as multijet and hadronic t¯t events) are estimated with DD methods.
To enhance the statistical power of the MC samples, the background with one fake lepton is estimated
with the loose lepton selection described above. A multiplicative factor of 0.063 ± 0.013, corresponding to
the MC probability for events with loose leptons to pass the signal region (SR) lepton criteria, is applied to
the final results in each of the background sources.
The dominant contribution to events with one fake lepton comes from Z+jets events, with a leptonic
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